how old are they ? if old enough to earn or contribute towards it then it can become a family project. i think explaining that its not a need (they have what to get to school on) but a want, lets think together how to get the money for, ie you can spare X amount and they get the rest. you dont get cleaning help for X months and they do it instead.. i think its a really good opportunity to do some money chinuch. lehefech, i think better to be creative together to problem solve - better than a no, we cant afford it. of course, if the problem solving = no, then at least you tried and they see that you want to help them.
ps i would add that my opinion is not to involve children in your finances too much, ie showing them your budget plan etc (maybe when theyre much older and mature) because then they start wanting to get involved and tell you where you should and shouldnt be spending which is not really their place