Burn

Contact Apple and tell them about it. But it’s never wise to sleep with power converters next to you. What they do inherently generates heat so they should always be placed where they have air flow around them to dissipate that heat. Converting household power to USB power can indeed generate quite a bit of heat if the device plugged into that converter is charging or being used and charging.

Charging blocks generate heat when they're working and should not be left on soft surfaces - including your bed.

A burn is not surprising if the block was completely covered by your body and/or the sheets.

The charging blocks are safe to use when they are used properly - plugged in to a wall outlet and away from soft spaces so the heat can dissipate.

~Lyssa

Make sure that neither you nor your daughters plug in the phone anywhere near where you might fall asleep on it. Ideally, you should be plugging them into a wall outlet. If that's not possible, use a U.L. rated extension cord and don't put it in your bed.

Based on that throw the brick away; it is clearly defective. It should not have gotten hot enough to burn you.

You should have that looked at by a doctor and contact Apple support.

While all of that is true, no Apple USB power source gets hot enough to cause a burn like that, and the shape doesn't match any Apple product. And Apple power sources plug into the wall; you have to go to a lot of trouble to get them into a bed. So I'm going with the hypothesis that it wasn't an Apple or an Apple certified product. And possibly that the burn was not from a power source at all, but was a convenient excuse for a burn from another cause.

That is not the shape of any Apple USB power source product. Who made the adapter that caused that burn?