If no one else is going to reply then I will try....
The vicinity of Pen y Fan is not ideal for wild camping, as it gets rather busy, especially on summer bank holiday weekends. There are few hollows for hiding your tent and pitching late is going to be after most 5 year old bedtimes.
If you are able to arrange transport the Mynydd Du would be more suitable. There is lots of empty space and there are lots of places to tuck a tent out of sight.
Otherwise, you might find it easier to start with a taxi and finish at the Storey Arms. Assuming you climb Pen y Fan on your last day the previous camp site ought to be within 2 miles of the summit and at least 336m high. You could probably find campable ground on Bryn Teg, or in the valleys either side if you don't mind a steep drop to get there, or on the ridge south east of Cribyn.
Last week I walked up Craig y Fan Ddu and noticed in the valley below to the right (SO059186) there seemed to be relatively flat spots near the stream. At the bottom of the valleys there are woodland tracks, waterfalls, streams. These might be more interesting to a youngster than the bleak hillside higher up.
There are flat spots on the ridge south and east of Fan y Big, with one or two sheltered hollows.
Along the main ridges there are no streams, so you might have to carry a day's worth of water. There are trickles of water on the bypass to the south of Cribyn and on the 'Roman' road that crosses Bwlch ar y Fan.
It's quite a tough proposition for taking a 5 year old and you might be better off choosing a more amenable venue. Maybe Dartmoor, where wild camping is officially allowed, so you can stop anywhere.
Being busy, Pen y Fan is relatively safe, in that there are sure to be people around if you need help. However if the weather turns nasty you don't want to up on the high ground with one so young. Tough military men have died up there in bad weather. I presume you have a dependable tent so you can bundle him inside in emergency.