It's "bid" not "bet".
Contact the seller and ask him to cancel your purchase. Be aware that he does not have to do this. Also be aware that he may add you to his Blocked Bidder List, which will prevent you from bidding with him in future.
If the listing is still active, you can Retract your bid, and replace it with a lower bid more in keeping with your budget.
BTW, your grandchild had to confirm his bid twice. It is difficult to do this "accidentally".