He was born in Constantinople, the child of Baldwin II and Marie of Brienne. Baldwin was the last of the Latin Emperors who actually ruled from Constantinople. The difficulty with the "Latin Emperors of Constantinople" was that they didn't have the resources they would have enjoyed at home. They weren't landowners living off rents. Baldwin's "territory" was essentially the city itself, and he did not have the resources to control the country around him, where life just went on.
Baldwin went westward to beg for money, asking Rome and France to help support him financially. One plan was to supplant the Marchioness Margaret of Namur (a sometimes independent state, now basically a city in Belgium) to have the Namur revenues. It didn't matter to him that Margaret was his sister. Baldwin didn't stay to manage Namur, however, and after it was invaded by the Count of Luxembourg, Baldwin sold the rights to his cousin, Count Guy of Flanders.
Baldwin left Marie and a regency council behind while he traveled Europe begging. In 1238 they sold the Crown of Thorns to Venice for 13,134 hyperpyrons. Around that time Baldwin got money from Louis IX of France in exchange for some other relics, of which Constantinople had many.
But this is about his son, Philip, and you can guess where this is going. Baldwin and Marie borrowed 24,000 hyperpyrons from Venetian merchants. The mortgage, the surety for this loan, was their son, Philip of Courtenay. Philip was five years old at the time. He was sent to Venice to live in the household of two merchant brothers. He was there from 1248 until he was 17, in 1260, when the mortgage was paid with the help of Alfonso X of Castile.
Mortgaging your child seems like a cruel act by a desperate parent. As difficult as it is to argue with that, as usual, medieval sensibilities were different from ours, and never more so than in the idea of a hostage. In fact, the meaning and practice of "hostage" is my next topic.