That you are on here asking tells me you have some doubts or concerns and you are right to have these. Others have posted the types of things I would urge you to consider. Maybe draw up an old-fashioned list of 'Pros & Cons' for your current job and the proposed one for your daughter.
1: 3 kids, full time, for only £700 - seems like an exceptionally low salary -for 40 hours you have each/all the children this is probably less than minimum wage.
2: The salary is already low yet you would have the expense of petrol, lunches, presumably activities and/or materials + any equipment they need or safety kit to install.
3: How long would this arrangement be for? What happens when all 3 kids are at school?
4: What if your daughter has more kids?
5: Would you need to be a registered childminder? Have other children to make the finances work?
6: Would you have a proper contract?
7: What if your daughter's working conditions change and she tries to alter the arrangement? Or has months when money is tight so tries to ask if she can delay or reduce payment to you?
8: This will change all relationships - yours with your daughter; yours with the grandchildren; your daughter & her children. The dynamics will be different. You will be their main carer.
9: What happens if you and your daughter disagree on aspects of care: the hours, if you or she are ill, meal times, after school activities, discipline etc?
10: 3 kids will almost certainly be tiring, demanding, and what if they each want friends over after school or you end up ferrying them to lets of hobbies. sports, activities?
11: Where do you stand legally if you are now their paid carer rather than grandparent looking after grandchildren?
12: You say you would get paid holiday for 13 weeks a year but what about sick pay, pension etc. What arrangements is your daughter making for those 13 weeks or would you be asked to have them as grandparent looking after them rather than paid to care for them?
13: When do you find time for your own appointments, shopping etc or do you have to do these with little ones in tow?
14: What about any evenings, dates, work stuff, training courses etc when your daughter wants to go out, be late home etc - do you end up with the kids for extra hours or additional 'babysitting'?
I just put the figures into the calculator...if the £700 was a weekly salary it works out at £17,50 an hour (approx £5.83 per child). If the £700 was a monthly salary it equates to £4.30 an hour (£1.43 per child)....either way it is not good! I believe babies and very young children generate premium income for most qualified childminders.
I would think VERY carefully before you commit to this as, much as you might love your daughter and the kids and want to help, this could be a recipe for a disaster and impact all of you in a negative way. Don't be rushed into a decision/ Think about the points people have raised on here and any examples of the plus points or pitfalls people have shared.
Whatever you decide, good luck