Nope, we don't need a duvet as well. But you know yourselves - are you very cold at night? if you have the space in the car, take the duvet anyway. Don't know the cot thig you're talking of, but with our 1.5 we had in a sleeping bag in a travel cot and then sort of put fleecy blanket over bottom half of him if we remembered. Take extra layers for DC and then go on guy instinct i'd say
we've only ever camped for a weekend but much prefer a duvet to double sleeping bag. But would take the sleeping bag to put over the airbed, the cold comes up through it otherwise, as we found out the first time we took a duvet!
Have only ever taken the kids with airbeds / sleeping bags / readybeds, so no idea re. the tent cot thing. We always used to put them in fleecy all-in-ones when they were younger and take plenty of fleecy blankets to go on top of sleeping bag just in case. Though they never seemed to get cold?!
For us, we have airbed, sheet, double sleeping bag and a couple of large fleece throws to go on top. And I always wear socks to bed as that makes all the difference!!
We put something under the airbeds too, have used both those foil-type emergency blankets from Poundland and fleece blankets and both seem to help insulate a bit.
That's always done us in late April / early May, so should be plenty for this time of year.
I'm with Lil all the way ..... lilo, sleeping bag opened up over the top, pyjamas, socks, bobble hat and 15 tog duvet! Have fun - I live in Devon - believe me the forecast is NOT true - it's blardy freezing at night!
We use double self-inflating mats inside double sleeping bags for a really good base. The kids have one to share and their sleeping bags on top, while me and DH go under an opened-out cotton sleeping bag (or yes, a duvet would do) and big blankets to seal ourselves in.
For babies/toddlers I swear by a 2.5tog travel grobag over clothes with a cardi over the top for adjustability. They roll all over the place so just throw a small blanket over the top whenever you wake.
another tip, which seems obvious once you know it... if you are going to change into PJs, put them on ahead of bedtime under your day clothes before it gets too cold. Then you just need to peel off your top layers. Otherwise you get cold changing and it's hard to warm up.
I always take a fleece to stuff into the sleeping bag with me. This is because although the bag is rated warm, I am fairly small and have a hard time generating enough heat to warm the bag iyswim. never put ds in a cot either - always just had him on mattress between us.