From what I recall, credit bureaus changed the scoring models a while back, so that authorized user cards now do not improve your credit score. Having said that, from the standpoint that the number of diversity of credit lines can be important, as is their age and balance to credit limit ratios, this will all help.

The question is whether you are sure that you want your minor child to have credit history as this can be a double edged sword. What happens if your child turns 18, 19 or 20 goes through a rebellious stage and decides to use his/her excellent credit history to apply for a bunch of CC's and runs up huge credit balances, for instance? You really ought to consider the benefits and the risks of this.