She shouldn't be needing to send anything at all. He's their father, it's his time, he ought to be providing everything they need for while they're there.
It's their first overnight in years and they are staying 5 nights. You expect him to have spare clothing, hats, coats, gloves, spare shoes, wellies, favourite toys, medicines they might need or anything else, for three kids for 5 days when they don't usually stay there? Completely unrealistic.
50:50 residency then fair enough. This isn't that.
Him not returning everything is stealing from his children.
Jesus. What an attitude. No wonder some of you have so much trouble co-parenting without conflict.
Him not returning everything is far more likely to be because a child dropped it down the back of sofa/radiator/left it under the bed, or it accidentally got put in the washing machine and forgotten about when it was time to pack up.
And if he doesn't even know what it looks like then it's going to be pretty hard to know what to look for, and to make sure every item goes back with them. He's not going to be as familiar with all of the children's clothes and belongings as his ex is, if she's the one who chose them and they are usually kept at her house.
Particularly if he has a new partner with children of roughly the same age and their stuff is knocking around as well.
I think what he's asked is sensible and reasonable. If the OP doesn't want to do it she doesn't have to, but it might be in her interests to cooperate, unless she's the sort who is really going to enjoy moaning until Easter about how she never got back her DD's gloves as her ex clearly stole them just to be a bastard.