I answered a question about the way the Norse (particularly Icelanders) handled fostering a month or so ago. If you have any follow up questions just let me know.

Fosterage was actually a super important social institution in medieval Ireland which features prominently in both legal and literary texts, where in the latter it is noticeable employed as a setup for dramatic acts (most memorably in the Táin Bó Cúailnge when Cú Chulainn is tragically forced to slay his foster-brother and best friend Ferdiad in single combat).

Fosterage was employed on almost every level of society which was intensely hierarchical, so that the children of free farmers all the way up to those of noble kings would be exchanged between households to be raised and educated by another family. Legal texts stipulate (although this might not reflect reality) that these children were to receive clothes and food that befitted their rank, with aristocratic children being given porridge made with wheat and milk while commoners were fed porridge made with water and oats.

Why would both commoners and nobles use this kind of social institution when it seems limited to the latter in your example? Simply put, fosterage created strong bonds not only between children and their foster siblings but also between their two families more generally. These bonds could be described as something similar to marriage ties in other contemporaneous societies; to have your child reared by another family meant you could count on their support in legal disputes, conflict and (at an aristocratic and kingly level) even warfare. I mentioned the slaying of Ferdiad by Cú Chulainn above which clearly demonstrates how alike foster relations were to blood kinship in the work's context - Cú Chulainn slays numerous enemies without remorse until he meets Ferdiad in single combat and bests him, whereupon the hero sings:

"Alas, O noble warrior! O brave Fer Diad! O strong and beautiful smiter, your arm was victorious. Our friendship was fair, O delight of my eyes!... Alas! my loved one, that you should fall at the hand of Cú Chulainn! Your shield which you wore against force afforded you no protection. Our fight ... our sorrow, the din of our battle. Fine was the great champion. Every army was defeated and trampled underfoot. Alas! O noble warrior, Fer Diad!

All was play and pleasure until I met with Fer Diad in the ford. Alas for the noble champion laid low there at the ford.

The divisions between aristocratic and lowborn children extended into the kind of education they actually received, which was compounded by their gender: children of lower grades were expected to be taught the means and techniques for the proper running of an independent farmstead (animal husbandry, general upkeep, food preparation and domestic production like weaving for girls). Aristocratic youths were given a more "courtly" education (although it seems they would have also learned the basics of maintaining a household) and were taught how to play strategy games and horseback riding. Sports and games appear to have been an important feature in education, possibly as some kind of preparation for combat which may be indicated by notes in legal texts regarding injuries and wounds sustained while playing them. Hurling (which exists to this day) and wrestling are clearly identifiable sports while the nature of others remains unclear from the texts.