Baby spinach & baby carrots can both be added easily without changing the flavor at all.
Spirulina goes over well here also, it does have a bit of a stronger flavor but it's somewhat sweet....and it's nice to keep on hand for days when we're out of greens.
For myself & DS, I've found we can easily add 2-3 cups of spinach & a handful of baby carrots to almost any fruit smoothie. Any other lettuce / greens, more than a handful will change the flavor, and I don't even like them w/ much in it....DS will not drink it if it tastes "lettuce-y".
Also - it goes over much better if you drink it too, then just give it to the kids.... DS & I will usually share one jar/glass with two straws.
If the green color turns your kids off, use blueberries, the smoothie will be purple instead of green.
Or chocolate - one of our favorites still is bananas, spinach, ice, & cocoa powder. "Chocolate Ice Cream"
Another one that's not too green is Watermelon, strawberries & celery (devein/destring the celery first!), and tastes great.
(We've been making green smoothies since DS was an infant; he loves them all now, and drank almost any of them from about 9 or 10 mos on....but went through a LONG phase in the middle of refusing to even take a sip of any that were green colored)