FYI a GT 26" wheeler in XS (15" c-t / 12.5" c-c) size could be ideal as they have a bent top tube for more clearance. I just sold one to a fella for his 9yo - the frame was so small compared to the wheels that it looked like a scaled down 29er
No reason you couldn't fit narrow tyres, rigid forks and drops with a riser stem on it for a more monstercross build.
PARTS FOR SALE:
vintage Brooks Professional saddle | Specialized FACT CX fork | Columbus fork | Reynolds 531 fork | Avid Arch Rival v-brakes | various 700c, 27" and 26" wheels | vintage Weco hubs (fixed/freewheel) | vintage Raleigh ladies frames + lots more!