The children's rooms is often the first place people start at when doing a de-clutter. Normally this is because its the easiest room to make an initial visual impact. They have lots of brightly coloured toys that if its anything like my house are normally all over the place, even if you have recently tidied up.
Most peoples first attempt is to buy some cupboard or box storage (normally from IKEA). While this does sort the problem with them being all over the place you don't save any room in the house, if anything you now have less room due to the storage unit.
We would always recommend having the occasional rift through to see what is actually used. There is no point storing a toy that is aimed for a baby for your toddler, it will never get played with.
There may also be other reasons you may not want to sell or throw away these items. Some may have been christening, birthday, Christmas presents that you have fond memories attached to. Also sometimes it just feels wrong to throw something out or sell a gift that may only be a couple of years old.
You also may have that underlying feeling that you may one day you may have more children and these toys will not only be a great source of entertainment but also a money saver! Hand me downs!
We have a range of storage units, the smallest being 25 square foot which is perfectly suited for this.