Honestly? I'd walk away and leave him to it, find something else to keep you busy while he deals with the kids.
Kids are bloody annoying, even your own. But if he won't deal with it then you can't make him. A lot of other people will tell you the same.
I have had to deal with two dsd's asking how much money Dp makes, how much we have in the bank, constantly wanting to know how much things cost that we have bought. It drove me insane.
If it's things like what's for dinner, I just say 'I don't know yet, will look when I have finished doing x, y, z'. If it's something where they really have no business asking then just raise your eyebrows and ask why they need to know?
There are far too many single parents (of both sexes) who treat their dc like small adults and dump far too much responsibility on their shoulders. They end up knowing the ins and outs of everything and really shouldn't be worrying about anything except what's for dinner.
Have a chat with your dp and say you are concerned if you think the dc are asking about age inappropriate stuff but other than that, let him deal with it.