I would make an appointment to speak to the headteacher and calmly say that you have heard about the incident and would like some more details and would like to know why you weren't informed. I don't think that would get you a reputation for being a troublemaker.
I would be very surprised if there is no one in your dd's school during the holidays. The headteacher is contractually obliged to be "available" every day of the year (except Christmas) so you should be able to make contact and ask for an appointment. If there is no reply to the school phone, I would suggest going to the school - you will probably come across a few teachers clearing out cupboards and so on, and you may well find the headteacher. If not, you could leave a letter in a prominent place requesting an appointment to discuss the matter. If the hearing is the first day back, it's really important that you are allowed to discuss it beforehand. I think it's bonkers they didn't let you know about it.
If you can't get hold of anyone at school, which I would think is very likely, try phoning your local authority who might be able to help you get in touch with someone.
You may have more luck speaking to someone at school just before they go back e.g. the staff go back at my school on 3rd Sept but children start on 7th. It is very unusal for teachers to not have a training day at the start of the year. Also staff may be in getting ready just before the strt. I doubt anyone will be around in a primary school now.
Good luck, I hope someone can help you, it is a rotten position to be in.
Write to the headteacher c/o the school, copying the chair of the governors (whose address/contact details may possibly be on school website?). Even if there is no-one physically in school all the time there will be someone there periodically - include lots of contact details and I am sure one or other will get back to you.
The only thing I can think of is that they are trying to get someone qualified (not a teacher) to question dd so as to avoid being accused of asking leading questions/putting ideas in her head - ie to make it more likely that the teacher will be prosecuted.
But actually it is crap and you should have been told. Big hugs to you ad poor dd. I would ring the local authority if no-one at the school.
I'm surprised this is the first you've heard of the incident or complaint. Be a bit careful about jumping to conclusions wrt details, remember you only have heard one account so far. Not saying it didn't happen though. Agree contacting the head(assuming he/she is also responsible for the preschool) should be the first port of call, failing that presumably the other parent has a letter with a contact name on at the LEA who you could speak to and might pass on information.
I am amazed that nobody has contacted you regarding this accusation. Just a thought, but have the pre-school got your correct address? You haven't moved and not told them? I just wondered if they had sent a letter to the wrong address. Have you changed your phone number? You need to chase this up and clarify what is happening. Is the pre-school run by the lEA? Some are run as private concerns but in the school grounds. Is the pre-school open over the summer? Some run holiday clubs for school children, so there might be people there.