TLDR; little kids (including toddlers) don't like boredom, but it is possible to teach them to make their own fun without relying on you for entertainment. If you do that, you'll have a lot more time for yourself.
I've got three young kids, and I'm in the same situation you're in. I'll spare you the long backstory of how I came to this conclusion, but over time I've learned that there's a big gap between the amount of attention young children want and will ask for (which is effectively infinite) versus what they actually need to be happy.
The real problem is they're humans, they don't like being bored, they don't know how to deal with it on their own, but they learn really fast that you are a great source of entertainment. Once I came to see the problem this way, I realized I needed to help teach and train them to self-entertain when they are bored. That turned out to be easier than I thought, and it helped _a lot_ with the problem of being able to get work done with kids around.
The best way I found to train them is essentially to create a suitable play environment (in our little apartment this is just the kids room, with anything I don't want them getting into placed on high shelves in the closet), set out a small sample of toys or activity (like paper and crayons, or stuffed animals), and then just sit there. If you are more boring than the stuffed animals, they'll start playing with the stuffed animals :)
Repeat this over time and they get much better at playing happily on their own for long stretches of time.
Note that the younger they are, the more you need to be physically around for this to work. If you just plop a toddler in a play pen and leave, they're not likely to make it very long, because they also have separation anxiety and don't like being alone. They need the reassurance of an adult (or older child may also work) nearby to feel safe. But once they feel safe, their only problem is boredom. So by being around but extremely boring, you help them learn to solve this problem on their own.
Being boring doesn't mean cold-smoldering the kids, just kind of politely humor them without really engaging. I've found that "taking a nap" is extremely effective - just sit and rest, they know what a nap is, they don't take it personally. Sometimes you even get an actual power nap in haha.
To some degree you also have to put up with whining and fussing during the adjustment period. They're likely to feel frustration that their entertainment source has failed. Also, doing this training will probably be boring for you too. But the silver lining is once they start to play autonomously it actually becomes very entertaining to watch!
One last thought: as far as work goes, I think you (and your employer) will also need to accept that a strictly focused 9-5 schedule isn't realistic. Young kids need help with basic needs (food, bladder, hygiene), and there's nothing you can do that will get them to chill out autonomously for 9 hours straight. But if you teach them to self-entertain, even a toddler can go for 60-90 minutes, which is enough to get a lot of things done (and bigger kids can go longer).
The way I see it, I can either get up really early and knock out a ~3 hour focus block before they're up, or stay up late and do it after they're asleep. I generally prefer late. That's my key productivity time, for things that require more than one full hour of dedicated focus to make good progress.
I know it's hard. Good luck!