Am I missing something here assuming he is 10 years old or so.
Erm why would grandma need child locks, he's not a toddler.
And the not being allowed on a bus or train .. Doesn't he go on school trips? He is going to grow up very sheltered and will be very naive compared to his friends.
After my DH had been seeing one another for 18 months we decided that we wanted out DC to meet. My ex wanted to be there, which was fine, DH and I simply picked him up from his dads when he was coming back to me, we stayed 10 mins and all was good.
My DHs ex actually said that was fine but she didn't want to meet me. Roll on a year and we were taking both boys away on holiday, DHs ex was dropping him off, so he called, said I was there and I would love to meet her before we all went away as one mum to another, she popped in all good. That was 15 years ago, and whilst we are not what I would call friends we both know we can call on one another if we have a problem and have done so on numerous occasions over the years .. She even buys my ds a birthday present, how civilised is that!
If your DP can take him to cinema etc, can't you go to the exs, only stay 20 mins or so and they go off and do your own thing for the day.
All seems very bizarre