Yes, my kids have spent a lot of time with grandma. Mainly my oldest. I was a stay at home mom when he was little and my mom
Would help out because I was dealing with AH drama. But I am a good mom. You'll never find me out on the town partying. I am with them every night. They tell me they love me and I them constantly.
Feel guilty because they get to spend time with grandparents who dote on them? Psssh. This is TOTALLY NORMAL and an EXCELLENT break for you, mama. Unless they are going 100 miles out of radius, I wouldn't even bother asking or telling your ex where they are. He doesn't seem all that interested about them, unless it means getting digs at you.
Just my opinion. I found that unless it was a major decision, my good faith efforts to keep my ex informed were not appreciated and were opportunities for him to get under my skin. Little things like an overnight at Grandma's house weren't big enough to require a phone conversation.