Does it have to be steel? If the little guy is going to be on a Hamax style seat they have their own bounce built in as part of the design so frame material will make little difference.
I might be selling my Boardman as I will soon be commuting on the Brompton and I can see it ending up never being used.
It's the CX Team model from last year. Aluminium frame with carbon fork, SRAM Rival 1x with hydraulic disc brakes.
Loads of clearance - I have 40mm slicks on it right now with full length guards and it has rack mounts.
I upgraded the wheels from the utter crap it came with to some lighter, less crap Cosine wheels.
If I do decide to sell you can have it for £500 and I can throw in a Hamax child seat in excellent condition too.