in laws think I should let my son spend the weekend with them.

So the other day me and my husband and my son went out to eat with his parents (his dad and step mom) and they started talking about how my son needs to seperate from me more.. he's 4 and a half and has only been away from me maybe 6-7 times over night in his life, that's not much if you think about it.. that's maybe 1-2 times a year. He's never stayed away more than 1 night at a time. Every single time it has happened he cried as hard as he could when I left him with them and apparently he doesn't get better after I leave, he keeps wanting me until I come back.  I have never wanted to leave him for the night, It was always their idea and them being pushy about it. But anyway, they think since he's going to kindergarten in over a year he needs to learn how to separate since he will have to when he goes to school, so they suggested letting him spend whole weekends and I'm just not comfortable with that. I don't even like 1 night away so I can't imagine two back to back.. and I know my son would not like it, he is the biggest mama's boy I know. He's very hard to seperate from me especially at night. My whole point is, going toschool is not the same as staying somewhere over night.. I wouldn't mind letting him stay during the day and picking him up at night but they never want to do that, they just want him over night or nothing. Sorry this is so long.