Travelling from London to Antwerp

feival

I need to travel to Antwerp in December between Xmas and New Year's Day. I am travelling with my wife and 2 young children (3 and 5.5). I have checked the route on the RAC planner and it is less than 250 miles so I am prepared for my first cross-channel drive. We are gong to a wedding so may be there for up to 5 days. Can anyone advise me on:

1) cheapest deals on the ferry and channel tunnel - A return on eurotunnel is currently £96, speed ferries to Boulogne is £68 and P&O £60. I think it is worth paying the extra for the tunnel?
2) cheap 5 day add on to my motor recovary policy
3) best route - the sensible options seem to be either dover to calais or ostende - the route planner says calais but ostende seems ok too.
4) A few people have told me not to take the ferry in December - the weather means it may not sail and if it does the journey can be very unpleasant?!