Does anybody have a good website that can help me in organizing my kids activities?

Maybe Google Calendar or Remember The Milk could help.

Backpack would be well suited also.

You don’t say how old your children are, but a physical calendar posted on the fridge worked well for me. You can buy or print free calendars that have lots of space for writing in the date boxes.

That way, the activities are highly visible, no log in required and, in my case, it helped them learn dates, what tomorrow means or in one week. Plus you can add other things that crop up easily.