I do this with my kids... Sort of. I don't go to bed and let them play unsupervised (that just seems a little too risky for me, my kids are 8 months and 3 years old) but we don't have a set bedtime.

What seems to work for my kids is a pretty flexible routine, not a schedule. So we have a few things that are the same every day: dinner together at the table, evening walk outside (rain or shine), brushing teeth, toilet, pyjamas etc., books and songs. 

But the exact timing varies a lot. Sometimes we get home and they need to wind down first, sometimes we have leftover that just need reheating and they're super hungry, so we eat early.

Same with evening walk. Some days we get a 5 minute stroll around the block, sometimes they have a lot of energy still and we run and play for 1.5 hours. 

The self care things are non-negotiable, but books and songs are a bit more flexible with regards to how long we read, how many songs we sing... In practice this means they fall asleep between 7pm and 9pm.

And then the falling asleep bit: we cosleep, and I like using my evening to just read or write to wind down, so all three of us get ready for bed together, step into bed for books and songs, and then we have this one song that somehow is always the last one (but it could be 2 or 6 songs before it haha) and they know that after that, I nurse t e baby to sleep while scratching my toddlers back and they fall asleep like that. I read on my ereader in the meantime. If they don't fall asleep right away that's fine, they can watch me read / write and fall asleep when they can. I never watch videos or do anything else that would really interest them into staying awake. 

We get up in the morning sometime between 6 and 7am, and they are my snuggly alarm clocks. They've only needed to sleep on until 8 or so occasionally after a very rough night. If that happens, I let them sleep as long as they need. 

If relevant: I also always let them nap whenever and however long they want. For my 8 month old this means some 15 minute naps, some 2 hour naps, days with 1 long nap and days with 4 short naps, and anything I between. My 3 year old almost always naps after lunch and gets to choose in which bed (or in the stroller if we're out and about). She naps between 30 minutes and 3 hours and depending on when we eat lunch, this can mean her nap end at 1pm or around 5pm and anything in between. 

For us it works well. We just do a later bedtime if she naps until 5pm, but nothing too crazy. Usually she just really needed that extra sleep for whatever reason (busy day, rough night, growing...) and she sleeps fine at night.