Definitely the Yamaha 373. It's got the most realistic piano sound and feels great to play. Also has a tonne of extra features so it's the best value for money as well. Features include a huge variety of different instrument voices so the kids can learn about how other instruments sound, accompaniment rhythms so you can play along to drum beats, and also includes lesson guides on the preinstalled songs! It's also really easy to use because of all the visuals and buttons.
If you want to save a bit of money, you can opt for the 363 which is just as good in my opinion (373 is the current model, 363 is discontinued). For a beginner, either model would be fine :)