What I think is the right $ amount is directly relative to our income.
So if we earnt 50K combined I could imagine $100 would be a lot or too much, especially having several children.
However, we have a higher than average combined income, so have more to spare than some other people perhaps? But I certainly don't consider it excessive either. We have bought everyone annual passes for the zoo and our daughters will be her Xmas Pres, she has a Dora hooded towel, Dora doll with a bathing set so she can practice while I bath our newborn, Duplo set in a box and a Fisher Price doctors set.
I dont think any amount is 'right' or 'wrong', I'm sure every parent gets something they can afford that they know their child will smile about.