If you are travelling with infants and taking advantage of sitting them on your knee (so not paying for extra seats) then don't bother paying extra to reserve your seats since chances are your seats will be changed and you will have a hard job getting a refund.
We flew by Thomson and decided to use their Select Your Seat service so we could be sat together. We couldn't actually use the online service since we were travelling with 2 infants so had to phone and a nice lady chose 'the best seats for travelling with infants.'
Anyway to cut a long story short our seats were changed (to seats behind each other) since we were originally allocated an invalid seat configuration for infants, and now we are having a job getting a refund even though the rep at the airport said we would be able to get one. It's not really the money (£34)- it's more the principle.
In the end the flight crew were great and reseated us in great seats since the flight wasn't fully booked (we had a row each on the outbound flight.) But that's not the point, it's that Thomson offer this service at extra cost when seats aren't guarenteed (in small print) and it's hard to get your money back if your seats are changed. When travelling with infants there is probably a higher chance of being moved due to regulations and in all likelihood the cabin crew will try and put you in good seats anyway so why pay the extra. The airlines are just trying to make easy money.
I have suggested they stop offering this to people travelling with infants.....
(or maybe we are just a rare case and most people don't even attempt to travel with 2 under 2's!)