They are calling all the shots on this - their child, their rules - but you hold all the cards. Your son can't keep taking every friday off and keep his job, so they either need your (willing) help or they need paid childcare, where they will NOT be able to make unreasonable conditions and they will pay properly for the service.
I do agree with Monica, early on, that this may all be a miscommunication somewhere & if you go to the discussion with you hurt feelings tamped well down, there may well be a good outcome.
But a poster said early on, "life isn't fair". Neither do the people who hold all the cards in life have to bow & scrape to others' rules- that's life too.
As for "who bought the child seat?" Unbelievable! whoever bought it, it was no use without a car and you bought the car - a rather bigger purchase.
The fact that you love your grandson and want a relationship with him, seems to equate in some people's minds to you using you unique, valuable, own life to be a slave to your grandson's parents