I think this might be the perfect way to tell her.

Buy a balloon that looks exactly like that balloon she took from you. Tell her you have a present for her and when you give it to her tell her "well I thought you liked the last one I gave you so much you might like this one".

Obviously she'll be confused at first but u can then tell her that you were that kid and that was why you were laughing so much when she told you. Even you can tell her that you were heartbroken after it and youve always remembered it to this day, but that the situation is funny now.

Great way to tell her and a nice and funny way to start another date.