I got £38 tickets last time and this time.
We had a fantastic view of the stage B, where the vast majority of the concert was being played
BTW for the OP, the only downside of Dublin to Belfast, is that in Dublin there is a better atmosphere all around the venues for larger parts of the day, in Belfast, most people tend to turn up direct to the concerts, unless a much younger teen crowd (usualy drunk teens, which doesnt tend to be at a U2 concert)
However in saying this, U2 playing Belfast is fantastic, they dont play here as often as Dublin, but you'll still have a great night.
BTW have a look online for the earplugs that you can have your child wear during the concert, they dont cut out the sound, but make it easier on the childs ears.