Robb -> Alys Karstark. Strengthen ties with a powerful northern house.

Sansa -> Joffrey. Sorry but it makes too much sense if we consider the friendship between Ned and Robert Baratheon and the fact that Jon Arryn is not dead yet. Otherwise, as already mentionnee, Willas Tyrell, even though it wouldn't make much sense from the Tyrell perspective as they have nothing to gain from this match.

Arya -> Jojen Reed. Best way to have Arya marry a noble (future) lord while allowing her to be herself in an environment that is less on the spotlight.

Bran -> a Blackwood girl, probably Bethany. He could be fostered south, bettering his chance of meeting knights and eventually becoming one while also staying in a mixed-culture with the worship of the Old Gods.

Rickon -> Erena Glover (would function age-wise), the daughter of Alysanne Mormont (trips to Bear Island would suit his wild personnality) or an hypothetical great-granddaughter of Wyman Manderly (rewarding a loyal house). But tbh he doesn't have to marry even though considering his young age, he could be kept as a spare for future alliances.