The HGI at Hatton Cross is often quite a bit cheaper, and not much more of a faff transport wise - to T4 it's 1 stop on the tube (paid) or a few minutes on a local bus (free). (From T5 it's quite a bit longer on the bus, so I'd probably tube it there)
From the T5 Sofitel to T4, you'd need to spend 10 minutes on a local bus, free but not always quite as frequent as you might want (always seems to be 2 at once!)
Or, stay by Paddington station, take the Heathrow express both ways (+ connecting train T2/T3 to T4 on the way back), and spend a bit of time on arrival with your child in Hyde Park (5-10 mins walk from Paddington) or around the canals in Little Venice (ditto)