I'm wondering if they're the same materials? Mylar balloons are foil-covered mylar. Is a safety blanket the same? I'm not sure.
In the past, I've reshaped foil balloons using a Clover iron and I've repaired one of those plastic-y character balloons the same way. You'd use parchment paper, not wax paper, to do this.
But, a heat sealer, as you'd use on bags, is what's used to seal small foil balloons since they aren't self-sealing.
Heat is the correct answer provided that the material is the same. What happens when you try to heat seal it?
18 inch Qualatex stuffing balloons. Hopefully if you live near a florist that does these, they’ll have them. I live near one that does. Also, you’ll need a stuffing machine that holds the balloon open.