You used we far too broadly. This is your opinion, not ours, keep it that way.

Spin-offs have nothing to do with the downfall of the series. There is no evidence that the series is going to end anytime soon. The only reason that Final Fantasy can't last forever is because, it, like all other games, will eventually be overshadowed by newer, better games and series. As stated, Final Fantasy is still one of the most popular series out there, so it's going to stay in the spotlight for a bit while longer.

By the way, Kingdom Hearts wasn't "child friendly." I used to babysit a 5th grader who had the game, he couldn't beat the Alice in Wonderland, but he could go through and beat Lord of the Rings: Two Towers.