Thanks again. I contacted the airport who say Transavia doesn't have an airside transfer desk so we'd definitely need to go out through passport control, check in, back through security etc (hand luggage only). Given transavia have said check in and gates close 1 hour (ish) before departure I don't see how this is feasible.
KLM should know when they sell the ticket that this is how it works with Transavia and I don't understand why they think it's OK to sell tickets with such a short connection in these circumstances.