Awwww I hate to say it but the little one who cried for 4 days is actually fairly normal. You can expect the little ones to have a hard time feeling safe and comfortable enough right away. It usually takes a couple of weeks at least to transition a child and for them to learn the routines and realize they can trust you and trust you to come back after nap.

I think there isn't a magic age for transitioning. A 15-18 month old might be easier to tranistion if they have a precise nap schedule at home but it isn't a guarantee. I have had sleep issues with getting 15-18 months and even 2 year olds to learn our sleep routine too. The only cure is consistency, routine and time...with a lot of patience. It will get better.
Congrats on your new dayhome