Internet: Explain it to me like I'm 5!

Hi! My family and I are going on a 7 day cruise this June. I will be off from work and my husband will be off from work. We will only need the WiFi to let our 2 year old watch a few shows to keep him busy (our 9 year old will be doing without internet) and we will need it to check bank balances, FaceTime family sparingly, and maybe checking emails. We are going on an Alaskan cruise so I know our phone service will work when we are at port and probably in Canada, so I don't see myself spending $300+ on internet for me and my husband. I want to purchase one plan and share but I really don't know how that would work. Can you please explain?