Tried to follow your plans here lap lap but have become confused.
Is your intention to do this itinery?
Leave luggage - Airport Express downtown - Bus to great wall - bus back to downtown - Airport Express to collect luggage - Airport express back to downtown - subway to hotel.
For the price of left luggage and those extra trips on Airport Express you can get a room at the AC Embassy Hotel in Dongzhemen for 400rmb which is directly across from the AE entrance and bus station.
You seem pretty dead set on doing things your own way but I would urge you to consider doing all this straight from arrival. I presume an early arrival means you've had an early start to make your flight, coupled with a 4 hour round trip on a public bus and climbing the wall. With a small child. You're a braver person than me!
Also given your flight arrival time I can't see you making it to the wall before 4pm which doesn't give you long there and from a safety point of view I would not take that bus trip back with the light fading on my own, let alone with my daughter.
If you can make it another day, the Schoolhouse Lodge runs a shuttle from the Kempinski Hotel for 132rmb per person ( 5 min walk from Liangmaqiao subway, line 10). It appears to only run sat and sun though.
http://www.theschoolhouseatmutianyu....school_bus.pdf
I could possibly arrange for you to store your stuff for free in Dongzhemen but you probably couldn't collect after 6pm as the place would be closed until the next morning. Don't know if this is of any use to you.