Both of my kids started BJJ very young, but stopped pretty early too. A combination of factors pulled them away and I wasn't going to force them.
For a little while my daughter was asking about different martial arts, and I just encouraged her to pursue whatever she was interested in. I gave my honest opinion, and of course admitted my bias for BJJ.
She decided to give BJJ another try and absolutely fell in love with it. Now she trains an insane amount that's only possible because she's a teenager and her body is able to recover. She puts in something like 16 hours a week (mon-thurst - 2 back to back classes, but Friday and Saturday are her favourite days - she does 4 hrs each on those days. She rests on Sunday).
She's made a lot of progress, I hear nothing but wonderful things from her instructors, and she's totally in love with the sport.
My son also started again too and he's doing quite well. He doesn't train as often as his sister (he trains a normal amount - 4 or 5 classes a week), he's got other competing interests at the moment (Rubik's cube, gaming, animation, and music).
Now when I go visit another area with my kids we all train together and honestly there are very few things as wonderful as being able to bond with your kids over how to properly choke someone!