My 4yo daughter gets cracks behind ears I think from taking tight ts off over her head. What should I do?

More likely Eczema. This is a very common place for eczema. I would advise applying a topical over the counter emollient such as aquaphor. If intense cracking or itching persists consider topical low dose steroid ointment, such as hydrocortisone. This area commonly will become infected, if oozing or redness occurs you need to seek medical attention.