Hi winnie. I'll try to answer some of your questions, and you can also check out our collated info on Hokkaido <click here>
1) the best time to visit Hokkaido for child age 8
if your child likes to play with snow, i'd say the various ice and snow festivals during late January to mid February are the best times, but it is quite cold, even if there are great onsens all over the island. another great time to visit would be around mid to late May, during the Shibazakura season. other than the beautiful flowers, it's warm during the day without being hot, and cool during the night (average around 8-15C depending on which part of Hokkaido you are), and it makes it a bit easier to pack clothes. it's mid-spring around that time so there's lots of flowers but the biting bugs are not around yet, and there's also lots of fresh vegetables that are just starting to be in season.
2) Any nice guest house ?
i had a great time at a minshuku called Mokuri
in Kushiro marsh and the couple running it were very hospitable and friendly, and they cooked some great home cooking for my friend and myself. it's a bit far away from train stations. there's a lot of nice minshukus and pensions around Niseko. you can take a look here
, or take a look at some of the choices around Hokkaido via Jalan.net
3) Nice food recommend?
Sapporo and nearby Ishikari are known for Ishikari-nabe, a hotpot made with salmon, and Chan Chan yaki, salmon and cabbage grilled on a hot plate. Sapporo and Asahikawa are known for ramen, my personal bias is for Asahikawa-style with Aoba
being the top choice with an amazing soup stock. Sapporo's also known for their soup curry, which is great any time of year but really warms the belly during the cold winter. Otaru, Monbetsu, Hakodate, Abashiri are known seafood ports with great sushi and grilled foods.
4) Nice places to visit?
other than the aforementioned ice and snow festivals, the popular ones are the icebreaker boat at Abashiri during winter(usually don't start until early Feb), Yoichi Nikka whiskey factory is just a little bit west of Otaru, Otaru's Sankaku market or other big fish markets in Hakodate and Sapporo are also great. Asahikawa Zoo in the winter's nice with the penguin parade. If you like nature, there's hiking on Daisetsuzan or around Lake Saroma or into Shiretoko National Park. plenty to see and do in Hokkaido.
5) Nice transport recommend?
Hokkaido's really great for rental cars, outside of some of the main roads, there's usually very little traffic. the only thing you might have to watch out for are deers and foxes crossing the road. (and sometimes rabbits, sables, and other small animals)
as for public transportation, the main train lines do take you along the main paths around the island, and for specific events, like the Engary Taiyo-no-Oka shibazakura or Higashi-mokoto shibazakura park, there's buses that'll take you right to the entrance.
hopefully that helps you get some ideas about Hokkaido.