The small steam with 4 drivers is easier to rail for small hands and rods/valve components where used are visual attractions, and a physics puzzle for a young mind to ponder.
Worm gears aren't for kids young enough to want to roll them unpowered.
If the gears worry you; they are cheap to buy. Staple the extra in a tiny ziplock to the box or something. (maybe a retainer, Ive no gear issue, but have cracked two retainer brackets)
A simple kiddie starter might get broken; sure. The replacement might be well cared for. (you know the kids temperament better than us) We got nearly no new or "great" trains from Gramps until until loss led to responsibility. Then the good stuff was bought or you got to run something special of his, etc. (Gramps: Oh no, you aren't going to break MY toys, go break more of your own. That's a wake up. Don't give in for a while 
)
The oldest will likely comondeer the trains so pick out a cool item or two for the others to use while waiting on the throttle.. (the operating news stand with dog peeing on a #90 button for the little one. The humor is ageless.)
