Hi FamilyFromSanDiego (where I also have family!)
and Hi Camping Girl, thanks for the kind words.
1) Best places to visit to get into Flathead Lake with small children would be either Wayfarer's State Park or Yellow Bay State Park. Wayfarer's is very close to the town of Bigfork, so it's convenient, however crowded at times with boats going in and out of the harbor. Yellow Bay is halfway down (about 14 miles) the east shore of the lake. It's got a great, safe, warm beach for kids. You could easily spend an afternoon there picnicing and enjoying being in this incredible lake.
As for cherries, you will likely just miss the peak of the picking season. This year they are unusually late, with expectations that the last week of July, first week of Aug. will be the harvest. You will see stands selling cherries when you are here, these are imported from Washington. Depending on the weather, you might get lucky and a few orchards near Polson where it warms up faster, might be picking (or U-pick, which is more fun.)
2) Family-friendly places to eat: Not a lot in Bigfork, given that the good restaurants are more fine-dining. Here's some ideas: Sun Mountain Pizza on Highway 35 is newly opened, and has really good NY style pizza, plus salads and ice cream. Definitely family-friendly and nice ambience for a pizza place. We get ours to go and bring it to Wayfarers for a picnic.
Brookies Cookies! This is a must-stop place, in the village of Bigfork, right on the Swan River. Go there and sit on the deck to watch the whitewater of the Swan. Last summer when we were there we watched a black bear in the bushes below the deck. They feature...what else...cookies!
Los Caporales Mexican restaurant just north of the town of Bigfork, at the intersection of Hwy 35 and 83, has good food overlooking a garden and the mountains.
Echo Lake Cafe for breakfast or lunch. There is always a crowd here and this restaurant proves that you can be out in the "middle of nowhere" and be successful if your food is excellent, also service and ambience. You will be happy bringing the kids here if you are not going at a peak time. Ask anyone how to find it, it's famous. It's on Hwy 83 about 5-8 minutes north of Bigfork.
Laughing Horse Lodge in Swan Lake (20 min. from Bigfork) has a great family-friendly, Montana style ambience, outstanding food, and is a gorgeous drive. Serves dinners and weekend brunches....call to make sure they can seat you.
Family-friendly in Kalispell: Nickle Charlies, on Idaho St (Hwy 2) across from Wal-Mart (sort of). It's where the locals go with their families. Decent food, fun decore, not a chain restaurant.
The Knead Cafe in Kalispell is also where locals go, food is eclectic, kids welcome, they have a deck for outdoor dining.
3) as for horse trips, I'll be checking these out myself in the next week. Can't give you any firsthand info, but check the tourist brochure boxes and you will find several outfits that offer day rides.
4) You can rent boats from Marina Cay Resort (I believe they still do it) also Woods Bay Resort, which is a few miles south of Bigfork. We got a great pontoon boat and took it out to Wild Horse Island and it was so fun. A-Able Fishing Charters is supposed to be one of the best around. They supply the gear, but you will need a fishing license.
5) Unique shops: shop Bigfork Village or downtown Kalispell or downtown Whitefish. Lots of places to spend money.
6) Rent bikes from the little shop next door to Brookies Cookies. Take them on the Swan River Nature Trail, or inquire at the shop about other bike trails. There are not a huge number of bike trails in Bigfork, but there is a great one called "Rails to Trails" just west of Kalispell, plus part of it is south of Kalispell. Not sure how much bike riding you can do with a 3, 5 and 7 year old.
7) Speaking of that 3 year old....I always recommend Woodland Park on the eastside of Kalispell. Bring bread to feed the ducks! What kid doesn't get thrilled (or a little frightened) at feeding ducks and geese. Also, a great playground there, many walking and biking trails, plus a public swimming pool. Could go to Nickel Charlies' for a meal, or try downtown Kalispell's old fashioned soda fountain and candy shop (sorry, can't remember the name) it's on Main Street next door to Western Outdoor, cross street is 1st.
More ideas: go to the Great Bear Adventure in Coram. Walk the Swan River Nature Trail a short distance from downtown Bigfork.
This list have about a week's worth of great activities. Have fun chosing which ones suit your mood and the weather, and hopefully your memories of being in Flathead Lake will be some of your best ever.
Enjoy!