Definitely the nursery years especially before the age 3 funding (if you have a child born after 30 March you have to wait until September which is most aggrieving - mine were born mid April and early May!!!)
When we had our first the fees were absolutely crippling. We had only bought our first house the year before so had barely chipped away at the mortgage. I was a solicitor but less than 2 years NQ earning a VERY modest salary. After mortgage, bills, basic food and commuting costs I had £15 disposable income a week. If my train was late a couple of times I had to blow it all on taxis from the station to nursery. I was so worried about money I sometimes cried myself to sleep over it.
I really don't believe it will ever be that bad again, let alone worse.
Age 3.5 - funding made things easier. Plus our salaries had risen; I moved firms. We started overpaying the mortgage.
Age 4.5 - school albeit with wraparound care and holiday club, school trips, uniform. Easier still.
Age 6 - I was a senior associate and had a big jump in salary. We also each came into a modest inheritance, increased mortgage payments again.
Age 9 - we had another baby just as my pay jumped again. We paid off the mortgage. By the time 2nd went to full time nursery it just kind of replaced the mortgage payments.
Age 11 - no more childcare costs for eldest. Yes he did some sports hobbies, more expensive clothes/trainers and gadgets but a drop in the ocean compared to £1200 on childcare fees.
Age 13 - I was made partner and salary jumped again. Youngest started school. No more nursery fees!
Age 14 we moved house and did an expensive 2 year renovation.
Age 16 - reno nearly completed. We're now concentrating on maxing our pension contributions.
Next year it will be driving and the year after that uni but unless we have a lot of bad luck I really don't think it will be anything like those hard early years. It's not just the nursery fees either, it's that our income was so much lower and outgoings so much higher then. But I guess that's the benefit of nursery years, it facilitates the career trajectory that would otherwise be lost (and which a lot of parents forget to count as a cost. )