Well, I only really know of our own group, but looking at the pictures on the Sunflowers website the activities look very similar!
Our children visit for 4 weeks in June, when they are out of school. The first lot came in 1996 so the organisation here is pretty slick now; they are dropped off at the daily centre on weekdays between 8.30 & 9 and picked up between 4.30 & 5, unless there's been a full-day outing when they often get back later. They do some English and lots of craft activities, see dentists and opticians, have various local outings and activities, and eat tons of fruit. There are 2 interpreters with a group of c 20 children, one of ours has been with them every year and she is fab.
Here you generally have the same child each year; they come first at 7 (occasionally older) and the last visit is at 12. Some people have had 2 or 3 different children to stay. How they fit in with your own family seems to depend entirely on the family and the child that comes (not veryhelpful I know!) - some are wildly successful and some aren't, but I think the children who stay in a house where there are other young children learn a lot more English. Ours understands quite a bit but doesn't speak much.
Our boy came for the 4th time this year. He's 10 now, an orphan who lives with his grandmother and great-grandmother, and is a really sweet boy. He doesn't particularly like football, which my DSs (13 & 17 during this year's visit) play all the time, so he tends to spend quite a bit of his time at home on the playstation or watching videos (he watched Matilda and a Bug's Life about 20 times each!) - I think if you get a child with siblings, who is used to playing ordinary games, it would help with fitting in with yours. Luckily this year there was another boy staying a few hundred yards away and they got together most evenings, which helped a lot (but they still spent hours on the playstation!!!)
Ilya was hardly homesick at all; the first year he came he did cry twice when we rang his grandmother (on a Sunday evening at about 7.30 our time, they are 2 hours ahead) but ever since he has been fine. Some are better than others with homesickness - also with things like general naughtiness and behavioural problems - like any children!
It might be a good idea for you to get involved with your group as a helper before you decide to host; but there are host families here with children of similar ages to yours and they do seem to get along really well.
We find it difficult to keep in touch - letters go astray their end