Thanks, our ds is only 2 so will be happy to run about and play in mud so not too worried about the play area. I have heard that Kielder Forrest is good and Boe Rigg seemed a good base to visit places. My dh loves his food so may enjoy the cafe!
I've been a couple of times and it's one of my favourite campsites near us. We stayed in the wild camping bit near the stream and it was bliss. The café is a bonus and the shower / toilet facilities are among the best I've seen. The kids play area isn't anything special but to be honest my boys were too busy building dams in the stream and cooking marshmallows on the fire to care.
We didn't find the midges a problem either and went in the middle of summer.
Yes, have stayed there, it is in a lovely spot but the standard pitches are quite exposed on a ridge when it's windy. It is more sheltered in the wild camping area that yellow mentioned. There is a washing up area inside with plenty of hot water and well maintained and clean toilets and showers. We went to Kielder and also to Housesteads and Chesters (Hadrians Wall).