Honestly, now i feel guilty as charged as i do it to my 4 mths old all the time though. Hiwever i would think it very much depends on the baby. Whenever i pinched/playfully my first born often cried and i woukd think he must not like it or it could also be hurting him. However for my second child, he loves it to the core. Each time i do it he starts laughing, cooing and giggling. It happens till his cheeks turn pink too. As long as baby is okay, i would think its alright? Otherwise maybe gentle reminders to your MiL might help too.
Not sure if this helps :)
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