Have they tried to talk about this situation? Has she refused to discuss it?
Is the youngest 20 months now? If that's the case, she was only 10/11 months at the beginning of the year? Are you the reason for the break-up of the marriage or the first girlfriend/partner since the break-up of the marriage? Do you have children yourself?
My stbx left me last year in the very, very early stages of pregnancy. I now have an almost 8 week old baby. The idea of handing him to my stbx let alone put him into the care of his girlfriend (who is not doing too well in the popularity stakes with our older children aged 2 and 5) is, frankly, far too much to contemplate at this time. I would suggest that perhaps for the mother, handing over her 10 month old to another woman was/is equally difficult to comprehend (far easier with the older children and that's difficult enough) and this may go some way to explaining what is going on? Probably worse if you're the reason for the break up of the marriage (not a judgement on my part, just my own feelings towards my stbx's girlfriend - I suspect my feelings are 'normal').
I know that this is an emotional reaction (so all you chaps out there being denied access to your children, please do not shoot me down in flames - I do understand my baby has to develop a relationship with his father and I will be allowing it, as soon as he either drops the idea of the DNA test or he signs the paperwork and we get it out of the way) but it is, with small children, a very, very real, frightening (lots of other emotive words!)....reaction for mothers.
I have suggested a three-way
mediation with my stbx and his girlfriend regarding the care of my baby (I find it hard to see him as 'ours' since I was abandonned in pregnancy, birth and he subsequently has denied that the baby is his) which I think is very reasonable. I have expectations which will need to be agreed to before he goes to their home. Mediation may be something your partner wants to suggest? Meeting you as well if she has any concerns? Allowing her to see your home, where the younger ones will sleep etc? I know, I really do, that this is a massive imposition, but it might go some way to appeasing her?
Anyway, I just wanted to throw that in. Sometimes all is not what it seems. We all react in ways we're not necessarily able to understand when faced with these things and it does sometimes help to try and see how things might be being felt from the other side. I apologise if it offends, it is not my intention.