This is tough and you have my sympathy.
I can relate totally my DD is 12 and my parents would take my DD out twice a year & that's it!! They've never offered to have her overnight. Its definitely had a negative effect on my relationship with DH as we've never been able to have date nights/weekends away etc...like our friends.
It still hurts but I guess after 12 years I'm 'hardened to it now' and my DD wouldn't spend time with them now anyway. Its very sad because they have no relationship now & my DD doesn't even like having to see them at Xmas/Easter.
Even now at age 12, if I need childcare for my DD, like if I'm working a long day & don't want to leave her at home alone, then I pay for childcare, like holiday & after school clubs etc. They are expensive & my DD often doesn't want to go as none of her friends go, but on the plus side, having to make friends with children she doesn't know has definitely made her more confident and sociable than many of friends who were looked after by grandparents every day after school.
Your children are very young & I remember constantly hearing how amazing other grandparents were when I was at toddler groups or in the school playground etc...
Now my DD is too old for school-pick up's/groups etc... its much easier as I'm not constantly being reminded of this fact.
Its very tough and feel your pain. Just remember you are not the only one in that situation and maybe one day your mum will realise what she is missing.