The difference is minimal. Work out the effective marginal tax rate on the amount above 100k. In their circumstances while they have nursery age children it’ll be over 90%.
Not putting the amount into pensions for 3/4 years is crazy. Effective marginal rate of course drops once nursery finishes and the calculation then changes.
50k of the 80k is taxed at a minimum of 62% before you’ve even factored in childcare.
3 years of 80k pensions between them gives them a 240k pot. Instead they are taking no more than 35k pa cash each year (less if student loans to pay) and at the same time giving up around 10k childcare per child.
That leaves you with a choice. 240k in a pension pot or 45k additional cash over the 3 years. You’d have to be pretty desperate for cash to take the latter.