If you are taking the train to London from wherever you live but then need to get to Waterloo, I'd suggest just splurging on a cab. More money, much less hassle.
I investigated this option but I'm thinking it would be worse than the train as it would take 1.5 hours to get to Waterloo from our home and we'd want to leave in the AM which means rush hour and possibly longer delays.
My baby wouldn't last that long in the car and would need to feed. The train at least allows him to feed the whole way if he wants!
But it also means a toddler loose for my husband to manage. But at 1.5 hours, that's about the time it would take for us to get to our local station, ride to Waterloo. So it's a bit of a wash in terms of time.
Now I'm worried about the installation of our car seats in the Paris taxis. Hopefully it will work.
The Eurostar is about 2.5 hours from Waterloo to Paris Nord - that's real similar to what we'd experience were we to fly. My husband wants to fly but I'm scared to, honestly. My baby hasn't flown before, I'm just plain afraid of flying.
Maybe if I bring enough fun snacks, books and games, my toddler could deal well with 5 hours total on trains. Of course, we have the return trip to think about. Isn't just a one time thing!