i cannot stress this enough do not walk out of the house
if you want to do the sell the house when the kids are 18yrs old, and put the money in a trust fund YOU MUST ARRANGE THIS BEFORE YOU WALK OUT
In marital situations walking out results in the loss of some rights, and once you leave you cannot go back.
Once you leave the courts will say, house is hers to raise the kids, you pay 50%+ of mortgage, you pay maintennace, you pay % of kids expenses.
If you want the sale of house at kids age of 18yrs old. A contract should be drawn up by a solicitor, it should be signed and witnessed, and it MUST BE DONE BEFORE YOU LEAVE THE HOUSE. Once you leave and ask her to sign it she'll simply say "No" this will elad to costs, strain and bother trying to make her agree.
Just be sensible and sort everything out before you leave. Stressful it may be at the time, for you family and the kids...but it will work out better in the long run.