I think a fun weekend away is a better test than your garden. Too many familiar things. Just throw them into a new environment completely. Kids are really adaptable, more so than parents 😉
We just started early with our lot, just to get to bike races/events and if you do it regularly – then it’s an exciting, but normal thing to do. Think we started at a few weeks for one and about 1 year for the other. Just tied a knot in their sleeping bags to stop them sliding down too far. They’ve also got kiddie thermarest copies from decathalon or somewhere. No problems with being cold. Lots of picnic blankets are a good idea too if your’s need some dry floor space outside.
And if you take your bike, never forget their balance bike/trike/scooter/trailer/ whatever wheeled thingy, so they can really get stuck in with your activity. Ours always associate camping with riding bikes now – all good!