The stock strategy is likely over time to generate high returns, but to fluctuate down as well up dramatically from year to year. If they will access the money in less than 10 years, some people would suggest diversifying their investments, perhaps by keeping a third of the money in a bond fund and annually rebalancing. Or one third US stock, one third international, one third US bonds.
Re transferring to them, I think Lucky Penny Acres is onto something because their tax rate is probably less than yours. If you are firm about giving up control over the money, transferring now or soon could save them (or you) some tax money.
Best wishes to both of them!
PS. Not to be nosy, but if they're the recipients, why is it in your name in the first place? Shouldn't each one's money be in an account in their name, to be used when they are of age?