I offered to match(1-1) for Roths for my kids from the time they started working(varied from 14-16) and continue to do so(24,27,30).

I did it to encourage them to save for retirement and get them started. I do it now because I can and would rather offer this than have them wait for me to die to inherit.

In other words they can't simply pull the money out and buy a something :).

Depending on your state, they have financial majority at 18. They are most likely to follow whatever example you've set concerning finances. If you plan to have some type of control based on the Roth, good luck.

Putting $500 in a Roth isn't much different than having the money in their investment accounts. It's theirs to do with as they please. In both cases, you will be impacting their taxes and choices so it would be good if they fully understand what you've done/are doing.