Five people in Singapore is where things get tricky fast. Most hotels cap out at 2 or 3, and anything on Sentosa that actually fits a family gets insanely expensive.
Being 1 or 2 MRT stops away is the smart move. HarbourFront and VivoCity area make the most sense since you’re right by the Sentosa line and everything else. Look at serviced apartments around there, or hotels that do proper connecting rooms. It’s not glamorous, but it’s practical.
January is peak-ish season too, so prices won’t be kind. Booking early and prioritizing space over “resort vibes” is probably the only way to keep costs sane.
Sentosa itself is great, but for a family of five, staying just off the island usually makes life way easier.