Holy moly at last a voice of reason.
Not minimising what has happened, but it seems people have not read the OP's posts properly.
Also wondered whether there are a lot of cultural aspects playing into this and responses on here - it's all very 'girls are innocent/pure' and 'boys are creeps/evil' - just no balance.
13-year-old boys (I've had them! and daughters too) are in fact often very immature and I'd say a 10 or almost 11-year old girl (assuming she is Y6 as OP says she walks from school) would be at a similar level of maturity as the boy. He is not an adult, he is a child for Christ's sake.
Boys do banter and use inappropriate language - most of us with boys who are on group WhatsApp groups have seen it. Tends to peak around Y8/9. Not saying it's right, but these are well behaved boys from lovely, caring families.
Sadly no government to date have banned access to porn for children. This is a great travesty.
But, let's step back a bit.
OP was sexually abused and so - rightly - would be on alert. Even if you try to be balanced when something so awful have happened to you, it will be more difficult to keep an objective view.
Setting the scene:
10-year-old girl goes into the 13-year-old boy's who was gaming with his friends online. Assuming he was wearing headphones as most kids do when they do gaming and didn't know she was there (those of us with boys will know how they can get hyper focused on the game). Perhaps it was a shooting type game, adrenaline-fuelled and possibly the boys didn't actually talk about sexual things (or maybe they did - unless you were a fly on the wall...) but used sexual swearwords. Not appropriate necessarily but not unusual and he was in his OWN room.
When he noticed the 10-year-old, he APOLOGISED for using those words and asked if she'd heard about it before. Presumably because he was worried she wouldn't have in which case he would have felt bad and maybe worried he would get into trouble for introducing new 'bad' words to her.
10-year old said YES, I know this stuff from SEX EDUCATION at school. If the boy just showed her some porn (again we are not sure) it's obviously not great, but they are still very much kids, boys too at that stage, and kids at that age find anything to do with forbidden stuff or sex/kissing on TV incredibly cringe worthy but might be something you'd share if you thought someone else had had insight. This is clearly not grooming, it was not even premeditated. Calling the police I think would be OTT.
By all means, comfort, reassure the 10-year-old she did the right thing coming forward, and of course great that OP mentioned to the boy's mother.
One thing though, the boy's mother wasn't defensive immediately, but apologised and said she would speak to her son.
She obviously did, he denied some of what she said but most was similar to the girl's story. Tellingly, the boy's mother afterwards suggested OP baby proofs 10-year-old's phone and that the girl may know more than she lets on. I can only imagine this is because the boy mentioned something the 10-year old said. That could be him deflecting but why are we 100% sure? If she walks to school with friends, they presumably have smart phones even if she doesn't. To assume that the 'perfect' 10-year-old is completely unaware is quite naïve.
I do have a pet hate of parents who assume THEIR child is completely innocent in whatever situation they might be and who would never look at both sides. Seen it happen before, not with regards these types of serious accusations but similar where parents say 'Johnny definitely doesn't have SnapChat etc, I've asked and he said he doesn't', when my son has shown the kid has a profile. Perfectionistic children do keep things from their parents because they don't want to be seen as not perfect anymore. It's important to be aware of this. The way the OP describes her own children and how she 'loves' the other children (but clearly doesn't), is quite uncomfortable and strange reading tbh.
But also, Tourette's is generally found to have co-morbidities mostly OCD. OCD is frequently related to now reading social cues correctly or being able to control impulses (same as tics), obtrusive thoughts - these can be so pervasive that the person creates memories which are not always factually correct. Studies on how those with Tourette's / OCD process memories slightly differently. OP says that her daughter has some emotional/decision issues - I'd like to know more about this before everyone jumping to conclusions. I'm not saying OP's daughter is not telling the truth but I think it's important to be balanced in reviewing.
Being caught vaping and lying - depending on what it was of course - is not unusual in teens!!
I do think the OP is doing the right thing stepping back, but I also worry about the one-sided responses on here.
Would hate any of those on here that scream 'paedophile', 'groomer', 'creep' of a 13-year-old CHILD to ever be selected for jury service. God help us!