Should I worry if my child rolls onto his belly while sleeping?

No. When the baby begins to roll over, this is a milestone of neuromuscular development/ progress. Although I am not aware of any research data, we believe that restraints to hold the baby on their back are not appropriate. And when able to roll, sids natural safety probably is increased.
No. If your child can roll onto her stomach from her back on their own you can leave them alone. This is all part of a natural progression in her motor development. You do not have to flip her over to their back. If she is upset when she gets to her belly you can help her out by getting her onto her back again.
No. Babies older than 6 months are able to roll over whenever they want to. So, parents need not worry about their older babies if the babies roll onto their tummies while sleeping.