Platinum and AC membership do not allow access into the Arrivals lounge, but I don't think you're asking about that lounge which is post-immigration (once you enter the UK).

That said, it looks like you have good news, in that BA to Valencia departs from Terminal 3, (most BA flights leave from T5). SO... your plan would work. You + guest get access due to Sapphire Status, and AC membership gets your son into the AC. If you had a connection to a BA flight in T5, where there is no AC, your plan would not work (2 would have access due to your Sapphire status, but no option for #3)