Mothercare does them, I got them for mine in the winter season, but they might still have them in, have you a boy or girl? I have one for a girl that is hardly worn. It's a pink bunny rabbit suit with bunny feet and tail, she only wore it about three times then had a growth spurt! I bought a new one in the next size up.
It's probably going to be quite hard to find one in the shops at this time of year. There are a few on ebay at the moment,mostly secondhand .You could try market stalls, car boot sales or charity shops. Ask your friends and family first , if they have one that's gone too small.( the number of times I've hunted the shops for weeks looking for something, only for one of my sisters or friends to tell me they had one all along I could have had.)
I swore by the sleeping bag version when mine were little, not like a sleeping bag but are sleeveless and zip down the front and you can undo the bottom to change bum as needed, come in various tog levels but even a light summer one which would feel like vest/bodysuit consistency is enough to keep them covered and prevent them kicking off bedclothes and then squaking, the warmer ones are good for outdoors and winter.
kids usually need the same as us and if anything I worry about them getting too hot as they can't regulate their temp as well as we can....so probably just usual jammies and socks on to keep feet warm, happy camping
Yes it's for a little girl, (although my son wanted one and cried when I told him he'd have to have Tank Engine one.) My kids wear them camping, and have this weekend. I find the temperature always drops at night. I haven't been camping yet where I haven't worn thermals and pyjamas at night!
I got mine for this very purpose on ebay. I would say better to take one and not need it, than not bother and spend the whole of the next day driving to the nearest big town to try and find one!
I find layer best then if you get too hot in the night, you can throw a layer off - works for me and the kids - ok they are usually in bed with me, but that's not a problem.