What can kids spend their allowance on? It's important to know - and communicate - ahead of time what, if any, restrictions will be placed on a child's allowance.
If you are going to require that the child set aside some portion of their allowance for savings and tithing or giving, be sure that is communicated and worked out in advance.
Other spending limitations should be considered carefully. Certainly a veto is always appropriate on any purchase that violates your family's principles or values. The same goes for anything that's harmful, unsafe, etc.
But beyond that, keep in mind that an allowance, and specifically spending an allowance, is an important opportunity for kids to learn money management without risking their food and shelter in the process.
So don't be afraid to let them make mistakes and learn the unpleasant, but important, lessons before they get out in the real world where they'll find the consequences of unsound money management to be much farther reaching.