I have ADHD and struggle massively with the routine thing, but it helps a lot.
I find what's best is first, break the day up by mealtimes and aim to do these fairly early to give me time to do it all.
Also, get everyone dressed in the morning so you are able to just go out without having to have negotiation of the century first as this exhausts everyone. In fact, DH gets the DC dressed and breakfasted before he goes to work. This helps a lot.
So there's mornings where we get up at the child's time, we have chill time while I caffeinate and de-zombify. They have TV and breakfast. At 9, TV goes off (lolol in theory) BUT the important thing is that I must not stay on my computer past this time. If I do, it tends to be game over for the whole day.
9-11.30 ish is then the morning session. We all have the most energy and capacity in the morning so if we're going out, it's a morning. I try to schedule in several out of the house activities per week. For the days that we are at home, I have a list of stuff we can do that is a bit more effort for me, like rice sensory bin, setting up the ball pit or obstacle course, craft kits etc. Or if they are happy playing with toys, I might do some household jobs.
Clear up if necessary and lunch, or grab something while out or head home for lunch. Currently I'm not doing great with the screens, as they like to watch TV while they eat, but I'm thinking of having a no screen during the day rule from 9-3.
After lunch I have a little one so I try to get him to nap. However he seems on the verge of dropping this. While he naps, or if not, I have a list of activities I can do with the older one - Lego building, writing/reading practice, board games, jigsaw puzzle. He might also play by himself while I have a sneaky relax/afternoon coffee.
From 3ish everyone in my house is in meltdown territory. So it's TV back on, activities that need minimal input from me. Again I've got a list that I can take ideas from. Colouring, reading books, call a relative, let them play on phone, movie, whatever.
3.30/4ish starts the evening slow wind down - dinner, DH comes home and takes over a bit, he might take them for a walk, more sedentary activities or dance party. Clean up (helps reset the room for the next day) winding down to bath/shower, teeth and bed all starting at 6ish.
Preparation helps a lot - a stock of easy but new activities like sticker books, clear out any old toys that don't get played with, consolidate toys so they are easy to get out and put away and find things. Lists of ideas. Set destinations on different days eg library, park, children's museum, whatever you have locally.