Hi lily, I have a 3yo and just almost survived the first year with a baby as well!
I try to explain things to my DS1 in language that he can understand and in ways that can give him some appreciation for how others are feeling.
At first when I had DS2 I let DS1 stay sleeping in his cot, this prevented him getting up and bothering me in the night, as like you've said in those early days its all you need just looking after the baby in the night!
But if you've already moved her to a BGB and she's been there a while then maybe a gate over her doorway?
It might put her off enough from getting out of bed if you explain about how you want her to stay in her room at night because when the baby comes mummy is going to need all the sleep she can get!
I should say that my DS1 is faaaaarrrrrr from a compliant child
and explaining has seemed to be helping when trying to get him to do something that I want. (Very difficult most of the time!)
Now I have moved him into his own BBB and he stays there. I told him I only wanted him to get out of bed when the sun was shining in the morning LOL!
That said, have you tried a grow-clock? With explaination that she can only get up when it's "time" because mummy is getting ready for the baby soon and needs her rest?
Or if you think she just needs reassurance in the night what about a night light (the grow-clock serves as one too) and/or a special toy she can talk to or cuddle when she wakes instead of calling you?
Or if she just simply HAS to call you then how about a 2way baby monitor that you can both talk to each other through? That way she can still "connect" with you but there's no physical presence and so it maybe just enough to help her drop the habit?
I'd start praising her now anyway for being such a big girl and how I'm sure she can do it all night by herself etc. I've found praise like this goes a long way when dealing with a toddler and a baby, you can make them feel more special without feeling robbed of their place in the family as "the baby".
Failing that, I've been known to say things like "please get into bed now so that mummy can get some rest too. We don't want a grumpy mummy in the morning now do we?" LOL!
