A prayer to teach a small child to memorize and recite before a meal is this:
For what we are about to receive,
May the Lord make us truly thankful.
You might beforehand tell them about the exciting time Paul was on a ship with soldiers and sailors, who all fasted and worked hard for two weeks to navigate the ship because they feared the ship was going to wreck in a storm! And Paul counseled them to eat for their health and well-being. Then he gave thanks to God in the presence of all before they all ate together with good cheer. And God saved them all, although the ship was lost (Acts 27).
P.S. Small children also like to clean up and wash dishes with you. Allow extra time. Make sure everyone has their apron on (or a tea towel tied around them) to hang up afterward to dry ;)