what is the most basic free form 3d "walkabout" game on WiiU?
Disney Infinity's 'Toy Box' mode is perfect for your kids, since they can freely roam around a level you created without any worry of time limits or enemies. You can build levels however you want, but the building feature may be a bit hard for your kids to grasp. So, ideally, you yourself could build a level for them where they could just run around and/or drive by obstacles you put down. Though, to make it more fun, you have to unlock more items to use, which means you have to play through the story level 'Play Sets', which may be tough for your kids.
The other downside with this suggestion is that Disney Infinity, before any holiday discounts, is normally around $70 for the starter pack with 3 characters/playsets & the portal to place the figures on. And beyond that, your kids may want to buy even more figures and power disks to unlock more characters & items in-game, which are about $13 for individual figures, or around $30 for bundled playsets.
Unfortunately, there's not much else on the Wii U library that fits the bill, but there's definitely a lot out there on PC that could help ease your kids into getting used to controlling a character on-screen. One that popped into my head is Proteus; Unlike Disney Infinity, which was 3rd person, this is first person view of basically an brightly colored blocky island exploration 'game' with no timer or enemies to worry about. I could have sworn that I saw that Proteus was coming to the Wii U's eShop, but I can't find any evidence of that right now.
As for other open-world free-form games with no countdown clock and enemies that's coming soon to Wii U: I cannot think of any other ones out there. Unfortunately, there's no new Animal Crossing announced for Wii U yet, and I think that would definitely be the game to suggest for you. Hopefully Nintendo comes out with one soon! (The free Animal Crossing Plaza application is totally different, however)
Hope that helped!