No. If you leave £100k to your kids, that is tax free. But that means you only have £225,000 nrb to pass onto your wife when she dies.
Thus if your estate is worth £1m, you leave £100k to your kids and the rest to your wife. That means your wife receives £900k and your kids £100 and there is no tax. When your wife dies (assuming she has no assets of her own), there is a £550,000 nil rate band. The remainder (£900,000 - £550,000 = £300,000) is taxable at 40%.
You might want some tax planning advice with the potential to make some tax free lifetime gifts........