Completely agree that your kids sound desperately overtired. We are also a low screen household and I just don't get the modern practice of sticking kids in front of nighttime cbeebies, it's awful!! There have been numerous research projects now which show that blue light from screens (TV, iPad, phones etc) are bad for sleep mode and yet still we stick them in front of it and then expect them to sleep. Plus TV shows are far more stimulating than a book where you have a static picture and a gentle parental tone. When mine were little I got home at 6 so nanny could go home to her children, and I dedicated an hour 100 per cent to the children, bath time and chats, milk, stories and bed. Luckily they were already fed, same as at nursery. Partner was rarely home to see them before they were asleep but their need for sleep was prioritised over his need to see them - he caught up with quality time on days he was around and at weekends. Only after they were in bed did we get into chores, dinner etc and it's a double benefit - the kids get your undivided attention, which is just what they need after a day in childcare, but also the routine goes much more smoothly because they aren't vying for your attention among your own demands for washing, dinner, etc. It also means you get your jobs done quicker once they are safely in bed.
To put things in perspective, my 6 year old still needs 12 hours sleep. Plus catch ups on weekends after a busy week at school and activities, plus extra sleep when she's growing or fighting off a cold. People will argue children are different but most kids need LOADS of sleep, especially at 18 months - mine were doing about 14-15 hours sleep a day with naps at that age. I have lots of friends whose kids go to bed at 9 and up at 6 and are plainly knackered - badly behaved, inattentive, yawning, stroppy, eating badly as a result which makes things cyclical as then they don't sleep if they haven't eaten properly. We are in an epidemic of sleep deprived children because of modern practices of parents working long hours, wanting to see their kids, wanting to co sleep, wanting to be liberal with bedtime and routines, and in amongst all that the poor children's basic needs get lost because parents don't or can't fathom that their children are just exhausted.
I would recommend you trial an hour after work dedicated just to them and a new bedtime routine. 100 per cent attention and getting them into bed by 7. It will take a week or two for them to to adjust but they will benefit in the long run. They are tiny and nursery/childcare is bloody exhausting at that age. They need more sleep and 100 per cent attention. You'll have your evenings back to sort out chores, food, admin etc and you'll have happier, more rested, more cooperative kids.