Having children doesn't mean someone else's time becomes more valuable than yours, but equally, not having children doesn't mean your time should take precedence over theirs.
Again, missing the point.
She just wants them to make an effort for her, 1-2 times a year and not always be the one who compromises and organises. It's a fair feeling, not feeling valued.
So, let me ask you. If you, with kids, had say a 4.30pm hospital appointment that's a little drive away. Would you arrange childcare? If you say, had a big birthday celebration in the city of a relative who wanted a meal out with everyone in the evening, would you arrange childcare? If you had tickets to see a concert of a band you utterly loved and really really wanted to go, would you arrange childcare? If you had literally anything that was important that you need to go to, you'd arrange childcare so you could go?
Bingo. Crux of the issue. The OP doesn't feel important to them, not even once a year.
While it's ok for children to totally change everything for parents, it's also ok for the OP to feel undervalued. It's not about monopolising time or who's time is more important or whatever, that's totally missing the point. It's about effort made.