When I started going out with DH he was living in a house-share and I was there a couple of times when his DD was there, but he wouldn't behave like I was his girlfriend when she was there.
At around 6 months, he decided to introduce me to her and I just popped in for a brew, said hi and then tootled off again. He was very clear from the start that the time he had with her was HER time and I was very much a spare part - except for the loaning of my car when he needed to take her places.
When she was there he wouldn't talk on the phone to me or text either, although he would text when she was in bed - however late that may be.
You need to talk to him about it as sooner or later you'll need to be introduced, but as someone who is at the other end - DBD is in late teens now - and remembers being in exactly the same position as you, I have the following advice....
- How he treats you now when the DCs are around is an indicator of how it will always be, so stand up to it now if you want it to change. I spent years with DH ignoring me when DBD was visiting, going out with DBD without telling me, disappearing on holiday without saying where they were going or even letting me know where they were going and him generally acting like an arse to me when she was around.
It is perfectly possible to strike a balance of giving children the love, time an attention they want when he sees them without completely ostracising his partner, but not everyone gets this right.
- Never, ever expect to come anywhere in the pecking order when DCs are around. He will have got used to the time he has with them being completely and utterly dedicated to them.
- Detach, detach, detach. (you may not get why I am saying this now, but you will at some point!!)
If I could go back and do it all again, I would have a word with myself about the emotional roller coaster that it was going to be and also have a word with DH about how I expect to be treated with respect when his DCs are around as it will make everyone's relationships easier. It would save a whole load of stress if I could do that!
Good luck!!