you dont need to have the same IP, this is the year 2020!!Would olds be able to do a family group subscription? If the grandkids don't have NES Online the family subscription is $34.99 a year and is supposed to cover the online fee for 8 accounts. As opposed $19.99 a year for a single account. Though I wonder if it's allowed for people not all in the same residence.
Also, you'll get access to a lot of free NES and SNES games with your subscription.