I Suspect that my Grandchildren are being Sexually Abused

I agree with Bluebell - children who have been physically or sexually abused clam up about it - usually the abuser has told them not to tell anyone about it. Then they become rather sad and withdrawn. I came across a few children like that when I was working. Certainly not reacting by dancing suggestively etc.
But it does sound as if they're watching inappropriate things.
But I can't see how you can do anything more about it, Willa, living so far away. Hopefully, after all the upset, your daughter will rethink what's happening to her children and take some action herself.

Speak to Social Services, I assume that this term is common to the US as well as UK. It seems to me more generally that if the parents, for whatever reason, are consigning their children to the care of others for so much of the time, they cannot possibly know what is happening to them at school, in after school care or in full day camp in the vacations.

That the children are acting the way they do is worrying, whether the cause is sexual abuse or uncontrolled and unsupervised access to sexually explicit or pornographic websites. Sexual abusers are subtle and devious and wherever the abuse is happening, it will be as hidden as possible. If they are accessing unsuitable online material then it is clear there is inadequate supervision of their online time and they are using devices that do not have adequate control software on them.

if nothing else these children are suffering from parental and institutional neglect.

Its understandable you and your daughter are concerned, especially when the extensive after school and holiday care does provide opportunity for grooming/abuse to possibly occur. However it should always be remembered that most sexual abuse takes place within family/ friends circles both extended and immediate and generally children who are being abused are threatened to keep silent.
Assume she has discussed her concerns with the school but if they had concerns they would have been in touch with her and her husband to discuss.
Could it be that they have watched inappropriate things online? How much time do they spend together as a family? It seems little and assuming you are in the usa, holiday leave is much shorter than here in europe.
Could they be attention seeking? Their behaviour does appear to be very overt and you are right to be concerned.

Obviously this is very concerning to you but just a word of perhaps comfort I do know kids watch some ridiculous stuff on their phones and iPads and I have seen quite small children copying moves that their idols use and make really inappropriately
Could they have assessed some inappropriate magazines of videos if their mother is sure there has been no occasion they could have been abused

Normally if children are groomed or sexulally abused they are very secretive about sex as they are usually warned or threatened to say nothing so the fact that they are so overt really points to them seeing it as normal and more likely a copycat reaction ( still worrying though)

If it is happening at school it will have been flagged up by the teachers to social services possibly

Someone needs to sit the children down and talk to them and ask them where / who they get the behaviours from

It definitely sounds like something is going on and it is natural you are very worried

It is not necessarily a product of sexual abuse but something could be going on for them emotionally

I think a call to social services may be in order due to your level of concern and ages of the children

My Grandson can be sexually inappropriate due to his Autism plus he has no idea about personal space etc and can be overly affectionate and 'touchy feely'

I know part of this is normal eg he does ike to have a fiddle with himself lol and being overtly sexualised is very common with children with special needs etc but the level of sexuality here may be a worry as it is getting worse?

I read this online if it helps? ;

''Some parents and professionals become concerned when a child displays sexual behaviour because they fear it may be a sign that something inappropriate has happened to the child. It is important to note, however, that although "some children display this behaviour as a result of sexual abuse, many children who display inappropriate sexual behaviour have not been abused."
To help parents determine whether or not sexual abuse is a concern, consultation with a professional may be appropriate. "If the sexual behaviour is highly inappropriate, or a parent suspects sexual abuse, contacting the local child protection agency is mandated by law''