you need to take MTR in sapporo station, ~30 min, then by bus ~1 hour, but it close ~ 4 pm, so you better go early in the morning, not difficult and it worth to go especially with kids.
a large park, around one hour bus from sapporo, both kids and adult will enjoy very much, in summer ,a lots of flower and children playground, in winter, lots of snow activities, for details, simply look into the website.
If the trip is only 4-5 day, it is better to stay in sapporo with same hotel, especially travel with kids, it will be trouble to change hotel.
If you go to 登別, beside 海洋館, you can also go to bear park, 忍者村, and 浸温泉, but you must stay for one night.
For shopping, Jusco is good, lots of goods and cheap, akachan is near sapporo station, both places you can buy baby's goods, no need to bring from HK, eg .diapper, baby food, bath basin (吹氣的).
actually, 5 days trip, minus the first and the last day, one day to takino park, one day to Otaru, one day in sapporo.