I will be facing the same dilemma next year when dd2 starts reception. My decision will be to stay at 3 days a week unless finances change and we need the money.
Reasons being:
2 days to drop off and pick up to keep in touch with teacher, chat to parents, see child in classroom environment. All very valuable.
2 days to attend school meetings or events without having to negotiate work
2 days kids can come straight home after school instead of childminder
2 (short) days to get stuff done around the house or to have time to yourself. It's much quicker getting stuff done, i.e. shopping, on a weekday, as well as leaving weekends clearer for family time.
only 3 days a week to cover in the school holidays
It's just my 5yo in school at the mo, and she really loves the days I take her in.
Don't feel pressured from work if you have a clear contract of your hours. And don't increase your hours as a trial because they might not want you to change it again. I think especially if you work 3 long days, you will benefit a lot from 2 days without work or travel, and your DC will too.