I assume your partner has his kids part time? I find that anyone who isn't with kids full time is like that. It seems that they just want to forget rules etc and maybe just enjoy their time together?

But I would talk to your partner. Just see how he feels. Yes he parents differently, but the kids still need to learn respect for theirs & other peoples property, at the very least.

Good luck with the 4 year old though... I have one. They're stubborn, pig headed monsters who seem to get a kick out of seeing just how far they can push you... every minute, of every day... Mine hasn't drawn on the walls for a while, but seems to think threatening me with this sort of behaviour is the way to play it when in trouble :/