A missile strike is a legitimate casus belli. If you're not a pacifist, that means it justifies force to achieve a military objective, which necessarily justifies killing civilians.
Whether that's "countless" or a few depends on the objective.
In general, civilian-combatant casualty ratios range from 1:1 to 5:1. They tend to be higher in urban settings like Gaza. The Chechen wars were closer to 7-10:1
The US estimates 15,000 combatants have been killed in Gaza. If so, we would consider 15,000 to 75,000 civilian deaths to be normal at this point.
I consider all civilians to be equal, so I'm not going to separate children for the same reason I'm not going to separate Palestinian Christians, mothers, teachers, or retirees.
Okay, let me put it this way- are you so firm in your conviction that all the child deaths in Gaza so far have been justified that you would be willing to say that to the parents of a dead child? Because I'm willing to find some so that you can tell them that yourself. I bet they'd even be willing to get on video chat with you so you can tell them, to their faces, that their child's death was justifiable.
So, are you willing to do that? Tell grieving parents that their child's death was justifiable because Israel is accomplishing its military objectives?