Your child is only 10? I think it will not let them e-file if they are under 16, in which case they need to file by mail.
Not sure if online allows a 10 year old to create an online account. If you use the desktop download you could prepare your own return and your child's return.
If your child owes tax it can be paid by check or money order in the mail or by using a credit or debit card on the IRS payment site.
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