I've been trying to post less but I couldn't not comment on your post, OP.
I think a lot of posters are missing the point. To me at least, it's the inequality and the favouritism that's upsetting you, rather than some expectation that your PILs have a duty to watch your children. And I fully empathise.
I have two DC, as does SIL. Her elder DC is 1 year older than my elder, with our younger DCs being the same age. SIL gets 3 days of childcare from MIL per week, we get 0. We all live within 5 minutes of each other, all work FT. My DH is very close to PILs and is always helping them with something or another.
My parents both still work full time, and are unable to retire in the near future. PILs are retired. I totally get that SIL had a year of childcare before us so had some kind of routine, but I would've thought MIL would say that to be fair, she would have to change the arrangement in the future so she could help us all. Perhaps give SIL 2 days and us 1. SIL could absolutely afford an extra nursery day if needs be, we're paying for full time nursery as there's no one to help.
It does hurt. Again, not because I want or expect free childcare. It hurts because MIL is treating her children differently, and is building a relationship with SILs DC that she'll never have with ours. And she's of course very tired from the 3 days of childcare, so when I try to arrange for her to see my DC, she's often busy trying to recuperate, which is fair enough. It just means she doesn't have a relationship with her DS's children (my DH), which honestly breaks my heart.
What's more, SIL's PILs live in another country, but visit regularly to see their DGC and constantly offer childcare when they're here. SIL has said on more than one occasion that if her PILs lived nearby, she'd have full-time childcare from GPs. Ironic that she mentioned that to me when she knows MIL doesn't help us.
To reiterate, it's not that childcare is expected. It's the difference in treatment. I couldn't ever imagine not helping my children out to the same extent. It wouldn't sit right with me.
Ultimately, you've just got to get past it somehow or resentment will chip away at you. I'm working on it.