As far as I know, you should be OK.
I think the difficulty will come if you try to remove the child now, or add another (within 6 months of the last change).

It may have been better to have added when you set up the HHA.

If you do a dummy miles booking, it should give you tick boxes against each member of the HHA - if the child is shown, all is well. (Did your child get an email to confirm their "invitation" to join, and have they confirmed it - in which case he/she should have his/her own BAEC number)

HTH