Rivendell is an Elven city. Frodo is a Hobbit, and much shorter than the Elves. The railing should appear much higher than it is. Was this simply a production gaffe, or is there an in-universe reason for the railing being so low?
There are a few possible reasons for this:
The room was specially made for Bilbo, and therefore everything was proportioned for a Hobbit
The room was a recovery room for Elves, and so the railings were lower so that they could see over them while laying in bed
Elves are much more agile, and might prefer lower railings aesthetically. They wouldn't really need them to protect against a fall since they're not clumsy
Peter Jackson simply overlooked it