Day Trip to Kamakura or Nikko

We're torn between taking a day-long trip to either Kamakura or Nikko. We're traveling with a 5 month old little one if that helps sway your thoughts. We're love the pictures of the waterfalls in Nikko, but is that going to be exceptionally hard to get to? Our baby will be with in a sort of backpack contraption, so walking is not an issue.

Thanks in advance!

When?

From where(which station starts)?

Assuming you are travelling from Tokyo and using public transport rather than a hire car:
Kamakura is probably easier with a little one, and Enoshima is a nice add on if the weather is OK to play on the beach for a while.
http://www.japan-guide.com/e/e3117.html
To get to the waterfalls etc around Nikko is not overly easy (we gave up) with trains and buses and traffic etc especially if you want to see the temples etc as well
It would be a lot more travelling in a day to Nikko and the waterfalls than to Kamakura
Enjoy!

We're actually heading to Tokyo next week and would look to take our day trip either Thursday or Friday.

Weekly(next 7 days) weather forecast update yet.

If not fine Kamakura better access convenient by train
and walk around flat most.

As much as I love Nikko I have to agree with AusTF and go with Kamakura for the following reasons:

1. Nikko is almost 2 hours each way from Tokyo vs 1 hour for Kamakura. On top of that you have to add the bus up the lake which during koyo season can be prone to traffic jams and/or long lines for the buses.
2. You may get up to the lake and waterfalls and find most of the leaves gone by next week.
3. Especially between chuzenji and yumoto you won't find many facilities to change a baby etc.

Both have their share of natural scenery and touristy areas but Nikko overall is more rural, so on the flipside if you prefer that (hiking etc.) you might want to go with Nikko, just be prepared for a longer day overall and try to get an early start.