help with MIL

so my MIL and her mum are coming to see us at the weekend so her mum can meet baby. They want to take her for a walk into town (30-40 mins) have a coffee and come back, without me or OH present and she'll only be a month. Am I crazy for being upset because she's still so young to be away from me?! she didn't even ask if it was okay she told us that's what she was doing.