Doge, why must you reduce this to absurdism.
Comparing a Women U12 to a Men's P/1/2 is just silly. The law would require equal pay for comparable events, so U12women and U12men would be the same.
Cycling is already moving in this direction at the amateur level in many places. I'd bet the bigger issue is in things like women's professional tennis.
As for the issue on field size, a simple disclaimer that the promoter reserves the right to cut purse by 50% for any fields under 10 covers that.
But really, the smarter move is to just remove the purse for amateur racing.