Another vote for the Wii.
Being an early adopter of a new system is pretty meaningless to your 6 year old, those of us who own a Wii U are either wanting to support Nintendo or are more dedicated gamers.
Wii U is a good buy if budget is no problem, especially when you consider it plays Wii games. Only downside with it being a new system is the available Wii U games aren't that plentiful, so the £200 odd price difference doesn't make that much sense unless you really want some of the current or future Wii U games.
But the majority of games that appeal will likely be Wii, the hardware and software are available at pretty fantastic prices new and used, it's also a very robust system with years of proven reliability. Really hard to recommend anything other than a Wii.