For raincoat, you might want to get a cheapie rain jacket like this
http://www.all3sports.com/product_in...200b54867ec360
if you can't invest in a better one. It will keep you dry for some time (but eventually will make you wet from the inside!!).

I agree with Denise about the shoe cover, although there exist some very good ones (expensive though) that can be used for riding in the winter. Bring a few plastic bags, they might come handy! (By the way, stashing newspaper in your shoes makes them dry faster.)

I'd take three pairs of shorts, just in case it rains one day and the first pair doesn't have the time to dry while your second pair is getting drenched...

(I live in the Pacific NorthWest, maybe I'm obsessed with rain?)

On a different topic...

Will your 4 yo kid stay with you in the tent for the whole trip? Have you travelled together like that before?

I'd suggest contacting the tour organizers about this and asking them to put you in contact with other people who have done the same with their child. I was talking with Grand Tour veterans last year (the Grand Tour is a week-long bike tour, very well organized, in Quebec, organized every year and attended by about 2000 cyclists - http://www.velo.qc.ca/voyages/travel...itre=grandtour for info) about this very topic after seeing a couple with a kid in tow. I was told that, as far as they knew, many had started the Grand Tour with kids below 12 yo in the past but none had ever finished. Indeed that couple didn't finish either.

I don't mean to discourage you at all and I know you are an experienced cyclist, so you probably have this sort of experience travelling with your kids. If not, do try to talk with others who might have tricks on how to make this work. I have no better suggestion, sorry...... (And, as an aside, I must note that as a kid I would have loved to do that with my parents!!!)