Yeah, I have the same kind of concerns because my 7 year old has been training judo for 3.5 years, BJJ for 1 year and is just finishing up his first season of wrestling. I have basically just been treating each of the sports independantly unless the rules specify something else (like naga won't allow wrestlers to compete below intermediate).
I haven't found judo to really give him that much of an edge in BJJ that he should have been anywhere except for beginner-- he's also big (74 lb.) for his age so ends up having to go against lots of older kids anyway, so it has kind of evened out. He's doing well but hardly dominating. He also runs into quite a few legit sandbaggers who have been competing in the beginner bracket for well over a year and even a yellow belt in the last one. His next BJJ tournament will be at intermediate but I expect him to be in for a bit of a rough go at it. In wrestling he's destroying everyone in his division with koshi guruma because (a) he's good at it and (b) wrestlers that age don't usually know anything other than doubles/singles so they have no idea what's going on when he does it. I definitely feel like a sandbagger (although I wasn't given a choice to move divisions), but if you see him try to get into referee's position or shoot a double/single you can definitely tell he's a beginner.