Given that they hadn't done anything wrong, if we didn't we would be no better than the nazis themselves.
But "we" weren't. It was the explicit policy of the Allies to kill civilians because those were part of the Axis war effort.
What was different about WWII was that when the Allies gained direct access to German children, it was over already. At the conclusion of the Dominion campaign in the Alpha Quadrant, the enemy in the Delta Quadrant is still in full fighting strength and shows no signs of remorse. Their one reason for not keeping up the fighting (Odo coming back) is alien to the Alpha winners, something they cannot really comprehend or rely upon. Killing all the Founders (and that really means
all of them, especially the children who would otherwise grow up angry, extremely armed and dangerous) would seem the "humane" solution, no different from how the Feds can steel themselves to slaughtering Borg and arguing they aren't really worthy of mercy or even capable of receiving it.
It's a wonderful piece of trust and xenophilia, fitting of Star Trek, that the Feds do choose to put their faith in the Odo solution. One just wonders how long that is going to last.
I just seriously doubt he gave a significant crap one way or another what happened to Odo
For Sloan to be consistent, I'd think he would be very interested in seeing Odo die, too - only not too soon lest he ruin the plan. Leaving a Founder alive, even a domesticated one, would be too much of a risk, as Odo might develop ideas, go to Gamma, and summon a trillion fighting men and a hundred thousand ships merely by doing the Neck Trick in front of them.
Timo Saloniemi