Rembrandt has posted what I was about to say - that as a first timer, single locks would definitely be easier. Whilst the Llangollen is lovely, it can be really busy even out of school holidays, and especially near the Aqueduct. The Oxford is really pretty, and also the Mon and Brec. We did a short break on the Mon and Brec to Brecon and back, that might be a good way to see whether you like it? We've never had a problem mooring in the evenings - as has been said, you can pick where you want to stop (as long as mooring is permitted there obv) and use your pins to moor.
You can use Canal Plan to help you work out a route and see how many locks any particular route might have, and how long it might take to get there.
With the boat, I guess the first consideration is the length. Obviously the longer they are, the slightly more tricky they are to steer, but you soon get used to them and everyone is really helpful on the canals if ever you're struggling . Some boats are fitted with a bow thruster, these are a bit easier to manoeuvre. The odd bash here and there is inevitable, we once took a brand new boat out and she came back with a number of bashes and scratches; the boat yard didnt even mention it.
There are various stern layouts for the boats, you can find out more here as there might be one sort that suits your family better.
Another thing to look at is the layout. We prefer a reverse layout which has the kitchen at the back and beds at the front so that in bad weather, whoever is steering isnt isolated from everyone else at the front of the boat. That might not suit everyone though as some people like to be able to sit inside and look out of the front of the boat
Internally, boats are usually well equipped and often have wifi and satellite TV. All have decent showers, microwaves, fridge and a cooker (we've even had a bath once!). For beds, you'll need to decide whether you want fixed or ones you make up/ a mixture of both.
Hopefully your children would really enjoy it - there will be lots of other children so it can be really sociable. Even walking to a canalside farm shop can be an adventure!
Sorry for the long post by the way. Oh and one last thing, if you do go, dont take suitcases. If you take squashy bags, they tuck away much more easily and can be used to collect provisions en route.