Also, starvation in children directly affects both physical and mental development, and thus affects the future of a population more so than when already developed adults are starving.
However, I do not believe we care more about starving children than starving adults. It’s just that biologically we are wired to take care of children, but not so much adults. Hence, ad campaigns for charities focus on the children as a way to reach our hearts and thus our pockets.