He couldn't ask them to care for a random child. He knew that straight away. Especially one that was enabled like him. Paige had recently taught him the benefits of lying if it served the greater good. He certainly thought it qualified. He was still suprised when she agreed it was their best option.
They ran the scenario through Ralph when they phoned him that evening. He unsuprisingly approved of the idea- he threw himself into the role of Walter's son with gusto immediately referring to Walter as dad and memorising every detail about the O'Brien family Walter had time to share in their limited but information filled call.
Walter was fairly concerned that Homeland didn't even bother to check that Walter was really Ralph's father- but he kept that thought to himself to avoid worrying Paige. A large part of him wished it was for real. That he really was Ralph's father and that his relationship with the young genius' mother was far from platonic. But those were thoughts he felt unable to voice as well.
He pulled himself together and continued to explain the childcare situation to Homeland along with providing essential information about Ralph himself.