We've both been working 80% since our first kid was born some years ago. We were earning very little for a while and daycare is super expensive, but last year we've finally started saving and investing. Having even just the two days at home makes a big difference for the weekends. During the week we are busy with cleaning, gardening or running errands, so it mostly doesn't feel like time off. But this means that on weekends we just do the work around the house that can't be done with kids underfoot, and that's it. Which is obviously great.
I would like to be done with the savings part by 40, which is in six years. From 40 to 50 we will let the stache sit and grow, reducing the working hours to just cover our living expenses, and then completely retire at 50. Actually, we will likely earn more than necessary in that period. We would be fine with just earning 40% (so, half of what we earn now), but this is probably too low to keep interesting, high-level jobs. 50-60% per person is more realistic.
We are not in a hurry to retire because a) we value our luxuries (living situation and trips, mostly) and b) what I would want to do after full retirement does not mesh well with providing a stable environment for kids. My spouse is not in a big hurry anyway.
After 40 we could of course easily continue working at 80%, or even increase to 100% as the kids are older. This would have a big impact on our savings rate and time-to-retirement. However, I can barely wait until scaling back as it is! Waiting until 40 is actually kind of a worst-case scenario for me. I have a set savings number (not percentage!) in mind, and my goal is to surpass it quite early (in 2-3 years) and once it does, I will immediately scale back work. This is actually not for the kids' sake (even now one of us is always home in the afternoon), but I want to have some completely free time for myself during the day, not just in the evenings when I'm tired anyway. Can't wait!