This would be better than saving in fiat because fiat is losing value year after year. Even if you save fiat in the banks that offer the best interest, it still can't make up for what your money regularly losses to inflation.
But this might not be the best option if your child is already in high school and you are investing in Bitcoin the money that he is supposed to use in college or university. You might get trapped in a bear market when the time comes that you already need the money. You might be forced to sell with a loss.