That's always a serious problem with kids programs. I tell people to avoid programs for kids that aren't run by people who have real experience working with kids. Those programs devolve pretty quickly, and it really is a totally different type of teaching.
My son has turned five, and REALLY wants to take Jiu Jitsu. There's a really good gym not far away, and while the instructor definitely knows his jitz... I don't get the impression that he really "gets" kids that well.
There are plenty of great blackbelts in BJJ who are much heavier, especially as they get older and stop competing. Carlos Valente (who I personally regard as one of the best living authorities outside of the Gracie family when it comes to jiu-jitsu) is older and heavier now. He doesn't compete and you'd probably mistake him for a flabby old man on the street. He's still an incredible instructor. I've worked with instructors who no longer seem to be in great shape. That doesn't diminish their ability as instructors.
Last night I found a closer gym, with good pricing. I talked with the instructor for a while, he seemed intelligent, knowledgeable, and very understanding of teaching kids. He's a black belt under Flavio Behring, and also has a black belt in Judo. In addition to two jitz classes each week, once a week, he also teaches the kids about things like "stranger danger", how to handle adult attackers and abductors, stuff like that.
There are some guys who keep the weight off just as a function of metabolism. My judo coach is in his 80s, doesn't run or do uchi komi or sparring anymore because of his bad knees and back, but he manages to stay under 180 pounds easily. Not everyone is able to do that. Not everyone stays as slim as Helio Gracie their whole life. It's really not a big deal.
Also, the guy sounds like a really good instructor for kids.
Now, here are the two things that make me pause: First, the instructor is close to 400 pounds, if not OVER it. He seems to have what it takes to be a good instructor, but that does make me question his activity.
In Brazilian Jiu-Jitsu, there are a ton of classes taught by non-blackbelt students. I would hazard to guess that, outside of Brazil, the majority of basics classes are taught by non-blackbelts, especially in areas of the world (and even of the United States) that haven't really seen a blackbelt at all yet.
Second, his daughter teaches the women's class. And his daughter is... a blue belt. I asked "You have a blue belt teaching the class?", to which he responded "She has over ten years of mat experience, rolling and striking." I've never had a class taught by anything but a black belt.
That said, bluebelt is a little bit low, in my opinion. Teaching classes as a bluebelt is precarious because it generally indicates a lack of competition experience. The purple and brownbelt levels are where it becomes pretty common to see people teaching classes. I'd watch the class and see if its something that you want to get involved in.
There are great classes taught by lower belts. I see it all the time, so don't let that, by itself, turn you away. But you are right to be wary.