There are potentially many ways in which your assets can be diverted from where you wanted them to go after you die. One way is that your spouse finds another partner who either spends your legacy or ends up inheriting it after your spouse dies, another is that your spouse gets duped by what are increasingly sophisticated scams, and yet another is that your spouse gets assessed for care costs and the Local Authority takes a chunk of your assets as well as your spouse's (and as the government has postponed the implementation of the care cap till at least 2020, this could affect a lot more of us).
To guard against these situations, you can insert into your Will a clause which leaves your half of the family home to a Trust which will ultimately pay out to your children when your spouse dies. Your spouse can stay in the house (or any other house they buy for themselves) for as long as they live and can carry on in pretty much the same way as they would if there was no Trust. The major difference is that your half of the house is now owned by a Trust instead of you, but significantly it is not owned by your spouse so cannot be assessed for care, used by your spouse without permission from the Trustees (of which your spouse will typically be one), or left by your spouse in their Will.