I'm utterly torn here. I can see both sides.
The employer shouldn't have asked whether the OP was planning children. They asked an illegal question and got an inaccurate response.
The OP is pregnant now, though. Whether that was planned or unplanned is irrelevant to the employer. Whether they should be told is a different matter, though.
I wouldn't tell my employer right now. I'd start training my junior and they'd probably work it out, and I'd eventually present it to my employer with a return date and the knowledge that I've already prepared my replacement for while I'm gone. There would be very little direct impact on anyone, really.
OP's situation is different, because they can't train a replacement. There's also the chance that the pregnancy will impact on the OP's job - lifting the toddler, for example, or deal with MS.
Then there is the chance that if the OP reveals her pregnancy, she may lose the job, especially as there are two months before the OP starts.
But if OP doesn't reveal the pregnancy, than that impacts on the jobs of the parents, who will have to take more time off to sort her replacement.
It feels like there must be a situation that is fair to the OP and also fair to the family and their children, but I can't see it.