Hello trinity, and hello tortoise
Where is the biggest problem trinity?
The easiest place to start is toys or clothes.
If you have a loft, pack up all the toys you can that aren't played with at least once a week, and by the summer you can safely get rid of 90% of i as the dds won' even have asked for i back
Throw away everyhing that is broken!
Go through the childrens clothes wih them and get rid of everything that doesn't fit/they don't like. Recycle, charity shop, or give to friends or sell it if you can.