I have had some more thoughts on this: John was born in 1266 which would make him around 16 when his father heads to Wales, John would make a name for himself during that campaign showing innate skill in fighting and command. It is said that john was the one who killed llewyn, and after much pomp and ceremony the prince is knighted. John would later be married to his cousin Margaret of Scotland with consent from the pope, allowing for a union between the two royal families and moving Edward one step closer to achieving his dream of a united Britain.
John and Margaret's marriage would produce five children: Eleanor born 1284, Edward born 1285, Margaret born 1289, John born 1291 and Isabella born 1293.
Margaret would accompany her husband on his travels and they would often split their time between both England and Scotland after Margaret is named as Alexander's heir by the estates in 1284.
John takes part in his father's campaign in Wales in 1294, earning even more credit to his formidable reputation by being the one to slay Madog in 1295, his father though dies from wounds taken during the battle and John becomes king, left with the troubles in Gascony and with the crown's own coffers nearly running out and tension in Scotland