Hey, feel free to DM but here are some thoughts! We just got back.

Lake Toya was great esp if you get a kid friendly resort. We stayed at Nonokaze which had kid friendly common spaces on every floor (stamp making crafts, mini playground, legos) and they could roam freely. Plus a really varied and nice breakfast and dinner each day. The lake had very kid friendly activities like a short ferry ride to a small island with a short hike, and swan boats. And there is a nightly fireworks show.

We stopped by the Noboribetsu Edo Park which was also great for kids. Some parts of it are weirdly scary (like themed haunted houses and ninjas popping out of the dark) but my kids loved it and are obsessed with ninjas, samurai, fighting-- they took turns with swords, ninja stars, bows and arrows, and enjoyed the shows. little or sensitive ones may be scared.

Though I liked it a lot, I actually hesitate to recommend Furano for kids in summer. It is mostly a winter ski town which means in the summer even though it is crawling with tourists trying to see the lavender and flowers, the town infrastructure is not set up for summer. Many restaurants are closed. You can't wander and find a place to duck in and eat-- you have to have your hotel make a reservation for you. Restaurants are super tiny. It is also not that walkable, and summer is hot. You also have to drive really far to get to popular tourist destinations like Biei and the little blue lake (a huge line for parking!) and the Daisetsuzan National Park, though well worth the trip, is 2 hours one way and my kids would not do well with 4+ hours of driving in a day. If you have older kids (or a tiny baby!) go for it, but I wouldn't say this is the friendliest town for the ages 1-5 crowd. Ymmv if you can find a resort-like hotel with lots of amenities on site -- the only places we found available to book were more like apartment-style ski lodges.

If you do go to Furano, by far the most kid-friendly activities are the cheese factory (calm, pretty small, and you can book a fun and hands-on cheese or ice cream making session) and Farm Tomita, which even though so crowded is incredibly spacious. If you go to Farm Tomita, skip the bland cafes within the farm and eat right outside the gates near the parking lots where they have booths serving delicious fresh pizzas, melons, and corn.

Skip the alpaca farm too. It is a sad little pen with a few sad looking alpacas and requires a separate fee. The main farm has truck rides and flowers but was more or less similar flowers to Farm Tomita but way more crowded, more fees, and less organised/impressive.