I think anyone who gives toys knows that they will be outgrown hun. I recently did the same and was reading a Montessori website about what kids need and the answer is, very little. Mine have their trains out right now, dolls with strollers, access to blocks and that is it. They are happy with those and because they are outside so much with summer right now they don't need so much.

Go through everything and throw out anything broken or from maccas etc.
Box up anything that you aren't sure about and put it away in a rotation box for 4 mths time or so.
Give away the rest to salvos and get the kids to help.
You will find it liberating for you, you will free up the clutter and space for you and the kids don't get so overwhelmed by the stuff around them(that is my experience anyway!) my book Organized Simplicity has really helped. I do it with everything,even at the shops now - I ask, is this thing USEFUL? Is this thing beautiful to me or us? If the answer is no then it goes or doesn't get bought.

Good luck hun and I hope you can sort it out relatively panic free