I think that you're misunderstanding the use of plug-ins. Neither of the things that you describe is a matter of just getting a single plug-in.
It's hard to know how to address your question without more context.
Are your kids already users of After Effects? If not, do you intend for them to be?
If your goal is to have fun with explosions in After Effects, then get them a stock footage pack with a lot of explosions and such in it. Then they can add these elements together, add them to their own scenes, etc.