BJJ clubs often don't work takedowns because they regard stand-up work as more dangerous. This is not unreasonable. Even many judo schools will have prepubescent students practice more groundwork than throws.
In particular, takedowns require students to pay attention to when they might get thrown, and execute a safe breakfall when they do. Being able to break a fall from many positions is a fairly complex motor skill that takes time to develop. Small children, especially those that are only training a couple times a week, often have quite poor breakfalling skills for years into their training.
There's a lot of curriculum to teach in BJJ: open guard, closed guard, mount, side control, back control, all of those positions in reverse, plus triangles, armbars, Kimuras, Americanas, naked chokes, and gi chokes. In contrast, a student doesn't necessarily need to learn throws in order to win--they can pull guard or get the fight to the ground by just...kind of...figuring it out....sometimes. (This approach favors the naturally athletic and aggressive and I therefore don't favor it.) Since kids have a lot of groundwork material that they absolutely will need to use, and the throwing material is not necessary to win in competition, a lot of instructors will leave it as an advanced topic.
3) Yes it affects competition performance
Students who don't know takedowns, or who have only done a class or two on them, will have to pull guard or rely on innate wrestling ability. This dramatically reduces one's options in competition, and leaves the student open to being dominated in the important first seconds of any match.