When do you introduce them, their children etc; to your DC?

[Tenrec]

My DC are 14, 13 and 13. His DC are 8 and 4. We have been dating for just over five months, get on very well, it's gone great. When would you introduce the idea of us/him? With his DC, I could see it as me being introduced as his friend, maybe going to the park or whatever with them like he or I would do with a friend and so on. But my DC are older and realistically, they would see right past that, in fact, even if he WAS a friend, they'd probably be a bit suspicious.

My DC have no contact with their father, and haven't done since my DDs were a few months old. They have brought up the idea of me dating/having a relationship before, but it's been very jokey as if they couldn't ever see it happening (which says a lot about me) but I've never said that it wouldn't happen or something like that. I'm feeling a bit lost, I wouldn't want to bring it up too soon, so have been thinking of introducing the idea in maybe a few months or something?

I think introducing it in a few months is a good idea. You say you've been together five months, which is still very much the honeymoon phase of a r/ship, so wait until you and your new partner pass the one year mark, and can be reasonably sure this one is a goer.
I think a very important part is talking with your partner first about what your expectations are re. each other's children. Do you/he consider living together in the future, or would you be happy for your relationship to continue as it is, at least until your DC have left home? Does he want you to take a step parent role for his children, and vice versa? How would you feel about disciplining each other's children? IMO, if you do decide to take on a step parent role, that is something that ought to continue even if the r/ship ends. How would either of you feel about committing to that?
If you do introduce your partner, don't pretend to your kids that he is 'a friend'. They will be able to tell from seeing you together that this isn't true, and it will start their r/ship with him off on a bad note.

Doesn't sound like there's any need to meet him yet.
I would definitely be telling then that you're seeing someone, and that you might introduce him over the summer, have a day out, if they fancy that. Give them a chance to get used to the idea.