Babies are cute if they're well-behave and are generally good kids. But since they haven't grown enough to learn how to express most of their emotion and needs, they will cry. Enabling that crying like Shiraishi is bad, but we should be more forgiving to children and teach them to be better. The phrase "they're just kids" means that they still have a lot to be taught.
But Seita isn't making that big of a fuss, toddlers can be a LOT more loud and whiny. That's because this is a fluff manga tho.