Have you thought of perhaps asking your MIL for her advice? Maybe bring up the subject by saying that having worked it out, you can't see how you're going to manage child care costs for two babies, as it's hard enough with just the one, and ask if she has any suggestions.
Just a thought, but it would certainly make her aware that you can't afford to keep paying her, and by asking for her advice, she might just decide to stand up for herself and tell your FIL that she's no longer prepared to charge you for a job which most grandparents would happily do for nothing.
As an aside, you don't live anywhere near me do you, as I'd love to be an unpaid childminder to your little darlings, my grandchildren are all grown up now, and I really miss the feel of a tiny chubby hand holding mine, or a sticky wet kiss just for being 'Nana'!
I do hope you can get things sorted out without too much strife, as being pregnant, all this worry is the last thing you need - maybe something your husband should point out to his parents if they don't see things from your point of view.