niece's birthday

Not sure of your age, but whatever, if you don’t want to attend, that is your prerogative. Listening to Boris last night, the party sounds like something that doesn’t really need to happen, but I suspect your sister does not want a disappointed child. It sounds like this cv will be around for ages, so lots of disappointment for many still to come.
Re present, no advice really, sorry.

You must not go if you feel that way. I am assuming you are considerably younger than the age group BJ and the government are talking about, but all adults can be at risk.
It is as simple as that.
My DD has just cancelled the party she and her fellow NCT /hypnobirthing mums had planned for next weekend for all the babies’ First birthdays.
We have not been ordered by the government to keep away from social functions but we have been strongly advised to.
Whether your sister postpones is up to her. In my experience little girls like things to wear (online shops such as Joules or Boden or M&S if you don’t want to go out.)
A subscription the the children’s version of The Week has proved very popular with my DGS, now 10

No! Your sister is been unreasonable and thoughtless. Don’t take any chances on your health. The on-line gift sounds ideal. Why don’t you send your niece a little card or notelet sending your love and telling her you hope she enjoys her party and you are sorry you are unable to attend?

Tweedle24 said it perfectly. Your sister is being irresponsible and the party is not an essential family gathering so it can be postponed or avoided.
Send a nice post card to your niece or maybe do a special birthday slideshow with music to wish her the best things and send it to her via email. If you want her to have something material, the gift certificate from the clothing or accessory store will do.

I think its unwise to go ahead with the party as well. At 10 years old your niece is able to understand the problems.
I definitely would not go.

Your name says it all OP!

Thank you, ladies! Thanks, @Esspee, that's so nice of you!
I was feeling down because I felt as if I'm letting my niece down but still she is a smart girl who can understand it.

As schools are still open at the moment then holding a party is not unreasonable if the parents are happy.

However, you should not go especially if you have a cough - no-one except your sister will want you to be there anyway!

Call or Skype your niece to explain why you’re not going. Send her an IOU for a grownup outing with you when it is all over. As for your sister.... nothing polite comes to mind!

You are right not to take unnecessary risks. Any one of those children could be a 'spreader' and if you got CV, you could pass it on to older relatives. It also occurs to me that grandparents could also have been asked to attend the party.

There aren't really any hard an fast rules so people just have to make judgements re being sensible and carrying on some degree of normality. I am going to a child's party this week but I would never suggest anyone else does. It is up to how you feel and what risk you feel you pose to other or what they pose to you. Mine is going to be at home and a pretty small gathering. I am not being glib and appreciate the seriousness but as nobody knows exactly what is going to happen we could be shut away for years! I do feel it for the children who are missing out on social activities.

@ValerieF have you not read the recent government guidelines about social distancing or do you simply feel that they don't apply to you?

I wouldn't go in your position.

Yes I have Granny Laine but as I said it is at the moment an individual informative choice. I would never tell anyone else what they should or should not do.

An individual informed choice?
Seems pretty clear to me that its not a great idea at this stage when the virus is spreading so fast.

Anyone who goes to an unnecessary social event/gathering atm is a selfish git. You risk contracting the virus ( or infecting others if you’re not in self isolation). If you are infected or infect others you are deliberately choosing to allow NHS resources to be potentially used when that could have been avoided and put lives and long term health at risk. Where there’s a choice, the answer is stay at home. And in most cases, there is a choice - a 10th birthday party ? Really ?

And the schools shouldn’t be open - the fact that they are is no reason to increase the risks out there.

I wouldn't go. All the schools in my area have had confirmed cases but none have closed yet.
I think that that my allergies are just playing up but am waiting to see if a fever comes on (some people are saying it was a couple of days after the coughing, others that they got a fever immediately) before I decide. We have to self diagnose at the moment but I am erring on the side of caution. The only place I think I could have caught it is either from the local shopkeeper or from a delivery man.
It takes very little. There is no point in risking it. No one should deliberately disappoint a child but it is unrealistic for her parents to think that she will never had to deal with a dis-appointment and really it is just one of life's lessons. It would be surprising if all the invited children's parents let them go.
Don't be guilt tripped. You aren't doing anything wrong.

Well Suziewoozie we all have our opinions but no need to be offensive. I am not a 'selfish git' never have been. I am just weighing it all up. Small house party in an area that is so far not affected with people who are as clued up and sensible as me. Obviously NO guarantees of anything but life does have to go on. Out of interest, how many children have the CV? I haven't heard of any. So I am guessing you will come back and say...give it time. Yes appreciate that but for now I will make my own judgement without being a selfish git.

Yes it is selfish atm to take unnecessary risks and actually tge word ‘git’ in no way expresses the strength of feeling I have towards people behaving irresponsibly atm. And if you bothered to actually inform yourself Val you’d find out that children get it, transmit it but hardly ever die of it. What an absolutely stupid uninformed comment. With a knowledge base like that, no wonder you are making uninformed decisions. Scary ?

OK Suzie, panic away. I will still be going to a child's birthday party this weekend. BUT as said I am not suggesting for one minute the OP does the same. We all have to do what we have to do.

I’m not panicking but your post is uniformed and demonstrates a selfish attitude.

And it’s not about individual decisions now but what’s for the good of the community .

The sooner we’re locked down fully the better