I have never seen anything quite like that... it actually looks like blisters... what is their heat source? Can you post pics of their setup?
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They have a tubby with heat lamp
How hot is the lamp? How close to them is it? It *might* be reflecting back up from the bottom of the tub...
Also, is their tub on a heating vent?
Blow dryer could have caised it too, lowest heat setting when using those...
Do you have any aloe vera? I would try that on their feet and see if that gives them some relief... or burn ointment? If an ointment, maybe a loose wrapping over it, don't want them eating it...