Margaret de Baliol

It has been established that John and Isabella had at least one child, Edward. However, other children have been linked to the couple as other possible issue:

Margaret Balliol. Died unmarried.

From Stirnet:

There is some confusion re the identity of King John's daughters. Various sources identify 2 daughters but they are named differently, albeit normally Margaret and Anne. Various sources (including 'RoyalData' and GenEU) suggest that the younger daughter married Brian FitzAlan of Bedale but TCP (FitzAlan) makes no such suggestion whilst RoyalData notes that it is not certain that Anne ever existed.

From https://en.wikipedia.org/wiki/John_I_de_Balliol (has in the generation before)

Maud (or Matilda) de Balliol, married to Bryan FitzAlan, Lord FitzAlan, and feudal Baron of Bedale. They were parents to Agnes FitzAlan (b. 1298), who married Sir Gilbert Stapleton, Knt., of Bedale [5] (1291-1324). Gilbert is better known for his participation in the assassination of Piers Gaveston, Earl of Cornwall.