Then fine if they don't offer it. It's your children, plan as if you don't rely on anyone, and if help comes, take it as a bonus.
They would be very mean if they stopped offering, they must like to spend time with the kids. Even if this happened it is ok. It's their life, it's your children, not theirs. Sorry to say that.
I think they decide on emotions, and then something else comes out, tireness maybe, so they change their plans.
It's free babysitting so just act as if it would never happen, and plan independently.
I'm a self employed, when I was younger I needed a place to run my business. Friends helped at their wish by offering occasionally a venue, but then cancelled, denied, changed their plans etc. I learned that you can really only rely on what you secure yourself, not favours.
I understand you are upset they don't take your time seriously and that's certainly disrespectful, however if you want total reliability this can only be offered for a fee.
As hard as it is I admit. I know it's irritating and generates resentment like they don't care about you and how busy you are, unfortunately not everyone is able to have this type of respect.
You can tell them though, on their side they're very rude, possibly they think it's ok for you so you can definitely set the record straight.