That probably should be fine. But the way to make certain that it is would be to make it a formal loan with a promissory note. Then there would definitely be no issues with reimbursing the child for the payment.
Harry S. Margolis practices elder law, estate, and special needs planning in Boston and Wellesley, Massachusetts. He is the founder of ElderLawAnswers.com and answers consumer questions about estate planning issues here and at AskHarry.info.