i agree with the idea of letting your children help with the 'culling' process. by giving them a limit of what they can keep for themselves (i like the idea of one box or bin each), and putting a limit on shared toys (legos, silks, etc), you'll be able to control the amount brought in to your new home.
when the toys are donated, i recommend that you take the children with you to the donation place. my mother made a point of driving us into *very* poor and run down neighborhoods and homeless shelters where we would donate our toys. we also donated to children's hospitals, and my parents took me there. THey wanted me to see how other children lived and know how i was helping these kids by giving them some of my things.
because of this, i often felt really good about 'toy culling' day. We did this about 2-3 times a year. We had to decide what was really important to us, and then we got the satisfaction of donating our toys to other, less fortunate children for them to enjoy.
I even remember an experience where i went to school and one of the little girls was wearing one of my old outfits (in good condition) and carrying one of my toys in her back pack. She didn't know that i'd donated them, but i did. I knew that she was poor and she was so proud of her nice, new clothes and her new toy. I NEVER mentioned that they were once mine, but i felt great pride in the fact that she was so happy with those things that i was able to give away.
So, this could be a huge growth lesson for your kids about learning to live with less, to love and enjoy and care for what you have, and offer whta you no longer want or need so that others may find joy in those things.