I wrote to Islington Parks dept and asked whether the park would still be open on that weekend, and they said that the concert was using 10 acres out of 115, so probably still enough space for non-concert activities.
Islington? Finsbury Park is in Haringey. The park may be quite big but take away spaces already used like the running track, the pond, the flower gardens, the cafe, the cricket ground, the road, baseball pitch and various football areas etc and it doesn't leave much space for Mum, Dad and the kids to sit and play.