They'll likely move in and out during the day and hunker under it at night.
If they never leave it during the day, they're too cold.
I find that chicks hatched and raised in cooler temperatures - provided they have a warm place and proper nutrition - feather out better than those kept cozy warm...
If you have electricity out there and they use the MHP, I wouldn't worry about them. They should be out around the clock. The fresh air will do them good.
A mother hen can easily raise chicks in those temps. All she does is provide a warm spot and lots of cool space. That's what your MHP is...