I agree with latexman, regarding setting an upper limit. I also would contribute to their retirement funds only as long as they are doing the same. If they are(say contributing to a 401K), then you could fund their tIRA or RothIRA annually. If the are contributing to a 401K, but not enough to at least get the full company match, then you might subsidize their biweekly takehome to their checking account, with an auto deposit, so that they could bump up their contribution enough to get the full company match.
"Luck favors the prepared mind"