Kids hold it together during the day at school and camp and daycare and grandma's house because they instinctively trust you to hold their tenderest emotional states. It's called RESTRAINT COLLAPSE.
Your attachment relationship is the place they feel the most comfortable expressing their strong feelings.
So many things happen during their days that they don't have the time or the secure place to feel and process, so those things come out at the end of the day with you.
But not always directly. Sometimes they go sideways about something seemingly small because they just don't have executive functioning skills to connect their emotional state to something that happened hours before.
When your kids get home and start melting down, offer them compassion and a safe place to be emotionally messy. And a protein snack. And be kind to yourself, you're not failing, you're their safe place.