Peter,
You might make the face from Das - if you don’t stick the smokebox door in, you could make the face with a “plug” that fits the smokebox “hole”, perhaps with a dab of Uhu, or even blu tak. That way, when he grows out of “faces”, he can pull it off, and put the proper door on, and have a “serious” model.
We built a Mavis (about 18 years ago, then about 3) for MasterD based on an Atlas chassis, and a Toby (about 14 years ago, as reported in the Gazette then about 5) for MissD, based on a Lima bogie. Didn’t do faces for either of them. Fitted both with cheapo Lenz decoders which ensure that Vmax was “safe”, I got them a Lima Toad, and a few Big Big minerals, fitted with decent wheels and oversize 3-links which worked well with little hands.
We still have the locos, though neither child is very interested...
All the best for the new year
Simon