During flight EZY8035, the 16.35 from Gatwick to Tenerife South on 18th July 2023, the seat (about 13c) of Sascha Butt (my son) was given to another passenger. After boarding and takeoff as normal, the seat was momentarily vacated. A passenger in the same row lay down and started to occupy the seat. Neither Sascha nor I were asked for consent or even informed of this action. All I could tell was a passenger who looked as though they were recovering from a heavy night and wanted a lie down to recover. To enable this, Sascha would have to sacrifice their seat for an indefinite length of time. I raised my issue with the aircrew. Marco was the most senior employee in the cabin. My concern was passed between two cabin crew then escalated to him. I was told that as I'd momentarily vacated, I had to maintain this status quo and sacrifice my seat. No alternative was offered. No timeframe was given. The only option I was presented was for both Sascha and me to squeeze into the same seat. It transpired the situation lasted for the majority of the flight. The FAA states "The safest place for your child ... is not in your lap". Yet this is the situation I found myself in and had to try to explain to my son. When I escalated my concerns, the best Marco could offer was advice I should write to customer services and request a refund for my son's seat. I've requested such a refund. It has been denied.