In my experience grandparents have often forgotten an awful lot about what it's actually like to parent small children on a day to day basis.
My mum has rolled her eyes when I haven't let her give my children popcorn or toys which are small enough to choke on and said, "I have raised children myself, you know!"
Well yeah, fine, but I'm not going to give my kids things which are unsafe just because you don't remember paying any attention to that stuff and I survived to adulthood.
Similarly, I think they've most likely forgotten the naps, the weaning, the potty training, all the stuff that you have to structure your entire lives around for a few years while your kids are that young.
It's never helpful to compare your kids' ability to cope with that of children you don't know. You don't know when those children went to bed last night, when they usually go to bed, when they last had a nap, how long for, or when they had their last tantrum. @toomuchfu You say your child had three 20 minute tantrums in one day which sounds like a bad day to me. But anyone else looking at your child outside of those three 20 minute periods might have been thinking, "See, that child is coping, why isn't mine?"
All you're seeing is a snapshot of their day, whereas you know what's going on with your kids all day every day.
Ultimately your mum isn't the parent responsible for your kids so if you think the holiday schedule and late mealtimes and bedtimes don't work for your kids, that's your call, not hers.