We are at Shotley Marina and have 8 year old twins, the kids love it as there is a beach, ice creams, mudlarking, hot chocolate all within walking distance. ( And a bar for the Adults ).
Before Covid, Alex had setup a Nintendo Wii in the bar to keep the kids ( and himself ) occupied, just don't beat him
There's plenty of space for them to play football, meet other kids, and generally run riot. We were in Woodbridge before and although there is a skatepark and swimming pool it's not really a good place for kid friendly day sails. From Shotley you can take a quick trip to Wrabness, Pin Mill, Stone Point.
In all I think it's the best place on that stretch for kids.