How much does it matter?
What you're looking for, rather than temperature, is signs of dire illness. A high temperature isn't necessarily a 'problem' in terms of health - but a relatively low temperature with a floppy, lethargic child who can't sit up to drink is. Delusional and hallucinating is a bad sign, but just having a temperature is the body's healthy response to fighting off germs.
Most medical professionals (after they ask for the number, they really are stuck on what they can write down, sometimes!) will readily admit that the appearance and behaviour of the child is far more important to the diagnosis than the number.
Frankly, an underarm reading, plus 2 degrees, is as accurate (with no risk at all of the always-deadly but rare 'rectal rupture' that is very difficult to achieve without a rectal thermometer -- in my opinion, that is way too much risk for a not-nearly-important-enough number!).
I never took my kids' temperature: I could tell when it was a bit elevated, and I could tell when it was a lot elevated. I didn't really need any additional information about the actual numbers.