If your child is small, hold them up to the sink. If your child can stand, use a safety step to boost them up to the faucet.
If your child is too heavy to lift and there's no step nearby, wipe their hands with a damp and soapy paper towel. Use another clean, wet paper towel to rinse soap off the hands. Dry the hands with a third clean paper towel. Wash your own hands after helping your child.