If your kids like apple and orange juice try this one-
4 whole apples
2 whole carrots
Juice them up and then it'll come out as orange juice. Then gradually increase the amount of veggie/fruit ratio.
Even let your kids pick 2 fresh fruits from the grocery store and one veggie you want them to try ( parents choose this if your kid does not want veggies)
make it a fun activity of making juice and then explain where it all comes from once they are comfortable on veggies and fruit.
I highly recommend watching "Fat, sick and nearly dead" on Netflix, its about benefits of juicing proven.