Peppermint and Spearmint
Peppermint is a classic herb for upset stomachs. It can be used to make a mild tea for children with upset bellies or used as an oil to rub (diluted) onto their tummies.
Spearmint is sometimes more of a preferred herb for children. It has similar properties to peppermint but a milder and sweeter flavor. Besides digestion, spearmint can calm hyperactivity and anxiety (especially when blended with lemon balm).
Either mint can be infused in hot or cold water, perhaps with lemon and honey for a “lemonade.”