S.D.
Breastfeeding made a great excuse to not let anyone take my kids until they were much older (or not to leave them here with anyone for more than a date), so I really don't know how that feels. When I sent my son away with Pappaw for the weekend, he was 2.5 and it was weird, but not sad or hard. Yes, I assume it will get easier if it happens more often.
To be honest, I feel better leaving my kids with my parents than my in-laws. I just know my parenting style is much closer to my parents than my husband's parents. So, my first choice is always my parents.