Kids this young don't burn out now. It's around tenth grade that burn out happens if you want to call it that. I call it something that might happen. Every kid is different and because that is the age when they rebel they will use the term burn out and blame their parents for doing too much for them. If your kid is having fun and you get to spend quality time w him or her driving to practices and tourneys then ride it as long as you can.
My oldest a junior in college just left the sport this past year. It was fun while it lasted and honestly I didn't expect it to last this long. One thing I always told him and myself was that this may be the pinnacle of your lax career and being a pro gets paid squat to play your goal is to use the sport to create friendships and to open doors that May otherwise be closed. Enjoy the ride it goes quick. Talk to your kids now because in a few years they may not listen. Listen to your kids too they will tell you of its too much