Adding my voice that in no way should behaviour like that be passed off as 'silliness' and not taken seriously. The boy took your dd's tights down and put his finger in her bottom! That's sexual assault. Alarm bells would certainly ring about the perpetrator's exposure to sex acts at such a young age too. Very disturbing.
FWIW I too have a 5yo dd in Yr1 - and there is no way on earth something like that would be passed off as 'cute' or just a bit of 'silliness'. Not all school cultures are the same, as in dd's school the atmosphere is very much on chase games, tag, and the boys/ girls play very well, in an innocent and non-sexualised manner. Last year in Reception there was a bit of "I'm going to marry so-and-so!" along with hand-holding, and sitting together etc. - but that seems to have passed into more sporty/teamworky games now. I do believe this is partly due to the school's way of emphasising respect and co-operation. It's an outstanding, inner city state school.
Anyway, I find your dd's school's lack of response - tbh, it's a complete negation and denial of your claims (as if just because the alleged perpetrators denied it, it needs no further investigation?!) to be pretty awful and unsupportive, more about covering their own backs. What messages are being quietly passed on to pupils here? It's a huge betrayal of trust, and it would not give me confidence in continuing to send my child there.
I would keep her off school until this is sorted and contact NSPCC for advice/support, SS, police even, if you get no joy from the school. Tell your daughter she has not done anything wrong, but the school needs to make sure it doesn't happen again.
I would be sending a letter detailing times, dates and details of what happened, and your concerns to the Head Teacher, board of Governors, copied to the Inspectors, and asking what course of action was being taken to prevent these incidents being repeated yet again (given something had already happened before). Yes - I would be causing a right stink!