Heathrow T3 to T5 connection- 2,5 or 6 hrs layover?

We are planning a family trip (children 7 and 11, not babies) to Chile, with a layover in Heathrow. The shorter European flight lands at Terminal 3, the transatlantic flight leaves from Terminal 5. It will be British airways, 1 ticket connection. I have two options. One is a connection time of 2,5 hrs, another 6 hrs.

It is one ticket so should we miss it, they would rebook. But the next possible flight to the destination is not direct, it's a connecting with Iberia, next direct one with British is in 48 hrs. So not ideal to miss it.

We would transfer airside, no luggage, just hand carry. Is 2,5 hrs to change t3 to t5 enough? Mid December, the transatlantic would leave at 22.15 so not the peak time.

Thank you for your experience/suggestions.