FunkyStickman is totally right.
We homeschool 3 kids, aged 4 - 10.
My general goal is to only fix what's broken. I.E. only buy curriculum/school stuff once a definite need is identified.
For instance, most kids learn to read by being read to and having the letters and sounds pointed out to them. Happily, library books are free.
Maths literacy is acquired through counting things, playing with shapes and patterns. Board games, card games, dice games will see you through here for most basic math concepts (and some other quite tricky ones)
Writing needs pencil and paper. Free printable worksheets are available online.
If you need some guidance about these three things, Ruth Beechck's 3Rs book is fabulous
http://www.amazon.com/The-Three-Rs-Ruth-Beechick/dp/0880620749Basically, I find it helpful to sit back, think "What skills do I want my kids to have 1 year from now?" "Will they learn that through our (deliberately chosen, intentional) daily activities?" "If not, what are my options?"
Very rarely will buying a full curriculum be your only option, especially while they're young.
Edited to add: I just re-read your OP and saw you were mostly asking about *online* options, not the full curriculum question. Sorry! No ideas, we don't use a lot of online educational stuff. If the kids are online it's usually to play MineCraft, or Hearthstone.