A simple dinner blessing for a young child to say can be adapted from the Lord's Prayer, such as "Give us Lord our daily bread," or "We thank thee Lord for our daily bread; amen." These mealtime blessings are ideal because they are short and easy for a child to remember.
For a small child, a rhyming prayer is also easy to remember. For example, an easy one can be "Father, bless our food, and thank you always for being good."
Older children may say mealtime blessing prayers that are longer. For example, an adapted traditional Catholic prayer before a meal is "Bless us our Lord, and these thy gifts that we receive today from your bounty. Amen."