Different anon, but I’m not sure it counts as fostering if you take on the kid as someone to execute in the event that his father attempts to rebel again. Sure, Ned was decent to Theon, but he was also purposefully distant from him for that reason.

In the medieval context, fostering is a legal process, it’s not necessarily an emotional one. Theon is called a foster sibling to the Starks and accepts that status as binding:

“Theon Turncloak,” someone said as he passed. Other men turned away at the sight of him. One spat. And why not? He was the traitor who had taken Winterfell by treachery, slain his foster brothers, delivered his own people to be flayed at Moat Cailin, and given his foster sister to Lord Ramsay’s bed. Roose Bolton might make use of him, but true northmen must despise him.