C.M.
I make green smoothies for my boys all the time (you need a good blender if you're going to do often)...
Blend raw spinach (I fill the whole blender lightly), water, 1-2 VERY RIPE bananas.. blend until smooth and liquid
Then I add froze fruit
pineapple (probably too strong for a 13 mos old)
mango
peaches
strawberries
It's a great way to get in a lot of green veggies, and fruit
You can add yogurt, probiotics.
I personally avoid adding juice... empty calories... all that fruit and greens are fiber... My kids have the healthiest poops you've ever seen... hard to beleive they come out of such little people!
Edited to say that I use sippy cups without the little plugs... everything generally goes through nicely...