1.

If u're within Sapporo, public transportation will do.

If you are going to the outskirts, better rent a car. It's worth it if you're going with a family. Can be even cheaper than paying for a bus to go to the outskirt (to and fro).

2.
I'm not sure. I didnt check it out when i was there. LOL. Either I was using MRT/trains or rented a car.

3.
Taxis in Japan are expensive. Might as well rent a car. There are taxi fare calculator for Japan on the net. U can see for yourself.

Driving in Hokkaido is not that difficult actually. The cars have English GPS and can use Google map as well. I'm at Hokkaido now and there's not many cars on the roads outside Sapporo. But then if Sapporo only public transport should be good enough

1. Very. Make sure you don't break any law because they have police all around. And don't forget to apply for international driving license.

2. What are the ones you've Googled? Mind sharing? Probably you should share which are the ones in your mind and ask for members' opinion and whether they've used these SIM before.

3. Sure, why not. As long as you teach them basic etiquette (no running, jumping into the water, splashing water etc). It is not a pool, so a lot of respect and no-nuisance is expected if you're bringing kids to the onsen. Others paid to enjoy some quiet time and relax, thus make sure they're not inconvenienced or affected by shouts, screams, cries. Although it's hard to control children but when they start throwing tantrums, time to get them out. I'll actually suggest you don't bring them in to allow yourself to have some relaxing time alone.

Onsen water is very hot though, see if your children can take it.

alternatively can buy locally from hello1010. click here