Problem is that if the net is too big, it will be different game. I was maybe too generic with "kids". Youngsters (cca 10 years and more) are playing in official nets on full size field, so if they were bigger they couldn't catch almost anything that goes directly into any of the corner, or they would have to be away from net extra few meters. This would change game drastically. Especially with young players I think it is much more important to build up chance, pass and "play", not only shoot slapshots from blue line. Also, in non-pro games, there are plenty of goals. There is no need to have much more. If you play on amateur level, like I do now, you would see that there is usually problem to find enough goalies (for many reasons) and with bigger net it would be even worse, because if you are under 190cm you would be there for laughs.