Ideas
I have four toddlers in my current household, and I have this problem with all of them.
I have my sim pick up one of their toddlers on the first floor, click on the toddler bed, usually on the second floor, and choose "read to sleep". Nothing happens. The action cancels out. My sims just stands around.
A workaround is to click on the floor, on the same floor level as the bed, choose "put toddler down here", pick up the toddler again, and then click on the bed and choose "read to sleep".
I have tested both with and without mods. I repaired the game and replaced the Sims 4 folder yesterday because of a different issue, and that did not fix this problem.
It’s definitely a new or new-ish bug. It’s been a bug when I’ve been playing for the last few days. It was not a bug the last time I played, roughly 2 months ago.