I'm a teacher and looked after my children in the holidays when they were young. I liked having that time with them but I did find pre school years harder for that as it gives you no time to yourself during the day, though mine are 5 years apart so I didn't have two under 5s at the same time.
When eldest was 2 to 3, she went to day nursery twice a week. I had to pay anyway, so I took her there as usual - plus it kept the routine going for her.
When youngest was 3 to 4 she had a childminder 3 days a week (part time, term time nursery) so again I continued to send her when I wanted to - she could go all day as I had to pay the full day rate even though she went to nursery in term time.
At primary school some years they ran a holiday club for two to 3 weeks at their school, so I booked both in for a couple of days a week.
Other years I booked them into clubs - eg gymnastics, tennis. Those were not necessarily for more than a few hours but it gave them a routine and me a bit of a break.
We have always had a holiday as a family in the summer- usually 10 days to 2 weeks long, or sometimes 2 separate weeks, so then my DH was with us.
Other than that I made sure I met up with friends and family for days out with the children. I tried to have 2 days a week either without the children or with another adult.