I've done it but pre-kids - and because I loathe flying, so I'm biased!
Second the seat 61 site - fab resource with loads of info.
I've done lots of training around Europe in the past - both with dh and without. I used www.railbookers.co.uk - including booking our honeymoon which involved trains to paris - spain - greece - italy - paris - home. They were great, spoke to them on the phone, got lots of help and advice and they even rang when a better offer turned up so we could upgrade our cabin for a small fee and get a first class sleeper which included a very nice 3 course restaurant meal and our own personal ensuite bathroom (which was small enough to sit down and go to the loo, brush your teeth and shower all at the same time should you so wish! but it didn't matter - it was just for us).
We also went to Belgium and the Netherlands last year with our dc - then 5 and 3. They really enjoyed it. One of the places we stayed in was Ypres (or Iepers more commonly these days) which was fascinating and a really good size to explore for a day or two without getting too tired out or overwhelmed but still having enough to see. If you don't know it, it's a beautiful old Belgian town but actually one where the centre has been mostly rebuilt (albeit as it was originally) as it was mostly gutted during the first world war. There are amazing war memorials as well that differentiate it from most 'typical belgian towns' and give it an extra dimension - that even my then 5 year old was able to understand a bit about and find interesting, and as he has subsequently done things like Remembrance Day at school he still remembers it and has had extra insight and understanding as a result.
One of the things we did in the Netherlands was go to a Center Parc - which I think was where they originated. It was great - less than half the price of the equivalent thing in the UK, more unusual things to do (the one we went to was called Het Heijderbos and had a full size dome dedicated to a big climbing adventure with rocky bits, rope ladders, wooden bits, zip wires, loads of tropical plants and trees, little brooks running through, even a few tropical birds and animals in it for authenticity. The kids adored it, do you remember the crystal maze programme? It was like a dome full of a jungle land to explore - a not so soft play massive area for adults and kids alike!
I went to Belgium and Holland quite a lot on holiday when I was a kid - and loved them. The cities, small towns and the seaside resorts were all lovely and exciting but in hindsight, felt safe which was probably reassuring for my parents as we were just at the age that they would let us explore alone for an hour or two before meeting up again.
Have a look at the site for Dutch Railways (in English). Costwise there may not be much in it but I think even with small ones you would find it a less stressful way to travel - and lots to see out of the window. You could always ferry it from Harwich to the Hook of Holland and train it from there.
London to Amsterdam is dead easy by train one simple change in Brussels and takes about 4 hours. I don't have a current idea of price, but factoring travelling to airport waiting time then travel from airport to city centre I actually felt the train was quicker. Eurostar to Brussels then follow large signs to Thalys platform then train direct to central Amsterdam. I am considering either Barcelona or Munich by train in the future.
We have holidayed in Amsterdam and all loved it. We are hoping to go again this year if we can stretch the pennies. I've been wondering about the logistics, price and experience of travelling from the UK (London, I guess, though we live in Bristol) into Europe and specifically through the Netherlands to Amsterdam. Has anyone done anything like this...? How was it with children? How was it cost-wise? Any practical advice weld be appreciated. In particular I'd be interested also to know of which are the best sites for booking etc as it all feels a bit mind boggling!! FWIW Easyjet could get us there for about £370 (that's two adults, one teen and two children and a baby). should we just fly, or let the spirit of adventure lead us a bit more boldly.....?! TIA. MPD