I'm sure we can't be the only ones who experience these problems when our children are this age. Or maybe it's just because we have kids in general.
I don't have kids, (I'm only nineteen! :p) but I have noticed that my brother brought on flashbacks last year because he was thirteen. He was also in another trauma of mine, so that was also triggered when he was around. I definitely had more flashbacks when I realized how great he was doing and realized that when I was his age, I wasn't anywhere close to how he was like in terms of a good quality of life. I think it's normal to get flashbacks when those we care about reach the age where we were traumatized- it's the idea of either watching them grow up happy during a time when we weren't (and maybe feeling a little sad about that.). And also the vulnerability factor- we realize watching them just how vulnerable we really were- When we are that age, we think we're powerful and can handle anything; so to see it through a different lens (i.e a grown up one) and watch how children that age actually act or how they really are- we realize that what we went through wasn't normal-and nobody should have to go through what we did.
It's really difficult getting through this. I get nightmares of my brother dealing with our trauma- I don't think he's aware of it. I think the best thing to do is realize that they aren't you. They are in a better situation, and they have you. Try switching to cognitive stuff. Stuff like "Just because he is the age that I was when I was hurt doesn't mean he is hurt." And then list all the things that are different for him. (He knows that he has people to talk to, he would know to tell someone if something happened to him (like for example, stranger danger- a lot of the time now, in school even, we have constant talks like "Don't get into a car with a stranger. If someone tries to ask you to get into a car, run to the nearest safe building and call 911.") He has several people that can help him if he needs it (teachers, you, etc.), if he was being abused we would know how to handle it by calling the police, telling someone, etc. etc) With my brother, I often have to remind myself "Yes, he is the age I was, yes, he is in a private school like I was- but that doesn't mean that he is being bullied. The school is not the one I was in. It's an all-boys school, not an all-girls one; and I am in Canada, not England."
I hope this helps. It's worked for me, but everyone's different. (I'm unresponsive to meds... so this is what is at my disposal.) It is something that you should talk about to a counsellor when you see one.