I agree, I think it depends on your son's relationship with the MIL. If your MIL is used to being with him, taking care of him, I say go for it. The first time I left my child with my parents (not MIL) was in Tokyo, when he was 4. He stayed with them for a week. We dropped him off and then we picked him up (he did not miss us much - ha). The first time he flew solo with my parents was this past summer, when he was 6 (layover, CA to Japan) but he probably could've done it earlier (I'd say 4). My parents are very close with him and visits/helps often - so it really depends. We also have 2.5 year olds - I think I would let them fly with my parents, but I worry more about random tantrums, etc.
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