Hey, our little girl was a touch older than yours when we first took her but this is how we kept her warm.
Card board of foam mat on the tent floor, picnic blanket on top of that with 3cm sim on top with a sheet. Then her regular duvet from home. We filled a hot water bottle to warm her bed before she got in then took the bottle out. PJs before eve got cold so she didn’t have to change when it was cold and try to warm up.
When she was a little older and we camped in a frosty April high on Dartmoor she had a jumper, a second duvet, a blanket on top and wore a hat on bed.
If anything both times she was too warm and rolled out of bed on to the floor, though she did wake once on the April trip cold so we popped her hat back on and covered her up and she was ok.
I’m sure you’ll have a blast and he’ll be fine. He can always snuggle with you if chilly. Enjoy your trip.
My children are older aged 8 and 10. We went camping last year in October and had the best time. We sleep on airbeds with plenty of insulation under the bed like foil backed blankets and carpet. We sleep in sleeping bags and the kids have duvets (with backup sleeping bags in case they feel cold)
Nice fluffy pj's come in handy and maybe with the age of your child could you get a wearable sleeping bag? Or maybe buy a good quality double sleeping bag.
We took a fan heater as we had electric hook up with a knock over switch just to blast a bit of warm air into the tent just before the kids settle down for the night.
When my son was younger we put him in an adult sleeping bag but tied cord tightly around it just below where his feet were so he couldn’t slide down into it on top of a SIM (these are much warmer than air beds). He wore a fleecy all in one with pyjamas and socks underneath (if the all in one didn’t have feet attached I put a second, thicker pair of socks on too). If it was really cold I throw a fleece blanket over too but this always seemed to get kicked off. He always felt toasty warm. We don’t use EHU so no heater but never had any issues.
Woolly hat and or hooded sleeping bag; insulated layer under him, self inflating or foam mat, foil backed or mylar survival blanket to reflect heat.
Air beds are poor insulators as the cold rises from the ground and acts as heat sink. Camp beds ditto, unless intervening layers as referred to above are put on top of them.
For our 18 month old we use a normal 3 tog baby sleeping bag (the one she uses at home in winter) we use the travel cot as it’s raised off the floor so cold doesn’t transfer up and add a blanket if necessary. We also bought https://www.next.co.uk/style/st214367
Which are fleecy and warm in case we thing we need a thicker layer.
I then check her when I am heading to bed by putting my hand on her chest under all the layers if she’s warm there then she is fine.
Our little ones first camping was at 12 months old. As above, we used a folding cot so not on the floor, 3 tog baby sleeping bags and some thick full body fleeces as nightwear.
Added a knitted blanket before we went to bed and no issues even when it was mid single digits outside at 4am.
Folding cot is bulky so may not be an option but you need a good insulator under them, air beds, even the baby ones are cold, not much better than lying on the ground and a lot of the ready beds have very poor insulation, made for a centrally heated house.
If you have little one in the same space as you, e.g. have the divider partly unzipped, that should also allow your body heat in the sleeping area to add a degree or 2 to the internal air temp.
If you have space you can always take a 500w oil radiator to keep the chill off.
Other thing to consider is warm milk in the middle of the night. We cheated and took a small microwave, if your using a pan to warm it, have a plan.... Nothing worse than hungry baby at 2am.
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