Wii U with Mario 3D WORLD

Wii U. Get some games with great local multiplayer and play with your kids. Super Mario 3D World, Nintendo Land, NSMB

A great 3DS game for your kids would be Super mario 3D land or New Super mario bros 2.
However, your kids would probably love Super mario 3D world and a wiiU aswell. It can also play up to 4 players at the same time (assuming you have atleast 1+ wii controller+ the gamepad).

Both are great options, it all depends on what you're looking for really.

I was around 3 when I first came in contact with video games, during the late NES early SNES stages.
With the proper education a controlled gaming environment can help improve early brain development.

I would go for the Wii U instead of the 3DS considering you are buying something for the family and it really promotes local multiplayer, something every family should be looking at as a major selling point.

A 2DS might also be a better option over the 3DS but the Wii U still tops this by a land slide for me.

wii u... because it is struggling :P

I'm kinda thinking Wii U mostly for the reasons you said in the OP. Also, I think 3DS is more targeted towards older kids/teens/adults because most of its best games (like Mario and Luigi) may be too complex for the 6 and under demographic

This. 

get them a WiiU /thread.

Seirously dude? Wii U? It should be a pretty easy choice.

You can always turn off the 3D on the 3DS or just get a 2DS for an even cheaper option

If you want to play with your kids / them play with each other then Wii U with Nintendo Land + NSMBU + SM3DW + Rayman Legends.