I don't think that follows at all actually. Every weapon has a balance of harm against benefit, if you outlaw cluster bombs why not mines? Why not grenades, or regular artillery? The reason is because the defensive value outweighs the potential harm. I think it's fairly clear that this is the case for cluster bombs too, while it is not for mustard gas.
The US keeps them because the alternative would cost significant capability. That would need to be made up for with other weapons. Politics and appearance costs impact things too, and for nations that could never stand a chance against russia/China without US help there is a much stronger argument for earning points by outlawing them.
The greatest risk to Ukrainian children is the Russian invasion, and the odds of Ukraine protecting them from that are far greater given these new munitions.