Saying no without being rude is a tact we all need to learn these days. I mean we should always help each other etc is true and a fact but these days people use and throw people like tissues so being able to say no when we feel we have done our best and are really unable to do more.
There's nothing wrong in saying no. There are times we need to be firm, be it with friends or kids throwing tantrums. All I would say is do it guilt free. This would help kids accept rejection.