This one is in the Compendium of the Catechism of the Catholic Church, and is the one I learned as a child (about 45 years ago):
O my God, I am heartily sorry for having offended Thee, and I detest all my sins because of Thy just punishments, but most of all because they offend Thee, my God, who art all good and deserving of all my love. I firmly resolve with the help of Thy grace to sin no more and to avoid the near occasion of sin. Amen.
Here is another that many children learn these days:
O my God, I am heartily sorry for having offended You. And I detest all my sins because I dread the loss of heaven and the pains of hell, but most of all because they offend You, my God, Who are all good and deserving of all my love. I firmly resolve, with the help of Your grace, to confess my sins, to do penance, and to amend my life. Amen.
And yet another which is found in the (approved) series of religious education textbooks used in our parish:
My God, I am sorry for my sins with all my heart. In choosing to do wrong and failing to do good, I have sinned against You, Whom I should love above all things. I have hurt myself and others. I firmly intend, with Your help, to do penance, to sin no more, and to avoid whatever leads me to sin. Our Savior Jesus Christ suffered and died for us. In His name, my God, have mercy. Amen.
I think any of these would be fine. The first two (and I suppose the third, though worded differently) explicitly include an act of “perfect contrition,” that is, sorrow because we offend God (not just from fear of punishment), and I think that is extremely important.