The granny should have taken the little boy to the dress rehearsal and then the mum could have gone to the adults only performance while her mum looked after the little boy.
One primary school where I used to live used to do plays at our secondary school. Babies and toddlers were banned but there was a crèche provided at the primary school about 10 mins walk away run by 2 teachers and several sixth formers (16-18 year olds). One mum was told she wasn't allowed to take her 2 week old baby in with her
. At the time I thought it was great (I was 17 and helping in the creche) as I got to cuddle a tiny baby for an hour but now I'm a mum myself I'm horrified.
I only caught a little glimpse of DS2 doing his play this year as I had to take DS3 and DS4 out into the corridor. If they weren't allowed in at all I couldn't have come as DS4 has complex SN and can only be looked after by me or DH.