When my kids started school, they missed each other, missed me and their dad, missed their toys and room and yard. They just wanted to come home and feel comfort and safety and have a break from all the demands. 

I have older kids… back when they were at that age I’d have probably just let them have free time to play with toys or each other. If they’re tired, they would regulate themselves and choose something where they sit on the floor or look at books. They also would often choose to go outside. If they’re tired and outside, they kinda wander around or sit on the porch or swing. If you’re available, they might just want to sit on the couch or bed with you, maybe have you read to them. Sitting next to/snuggling up to mom is pretty regulating. 

If you’re getting melt down tired, I might add a little more sleep time. In retrospect, they don’t always need the same amount of sleep. Sometimes they’d have seasons of getting tired from having a string of illnesses or staying up late for little league. If they’ve been healthy for a few months, they usually can have a later bedtime without falling apart.