Kids resources on the web

While the web is a great resource on any subject matter under the Sun and on stuff on the Sun itself, it woefully lacks resources on kids matters. I have a hard time finding out at what age kids should be proficient in some matters like say addition.My daughters are 3.5 yrs old and I think they should be versed with addition by now but my wife feels that addition is suitable only at age 5. They are very good with numbers and can count and write numbers, alphabets and 3 or 4 letter words but they just have a hard time with addition. Granted that every child is unique but a general guideline as to what you can expect from them at what age would be still a very good resource. It is surprising that there is not that many websites that guide you as to what you should expect from your child.

If you do find such good resources please leave those URLs in the comment section. TIA.