Re disco parties - this might be a bit late as you might have had your party by now - but my DD declared she wanted a disco party when she was 4. I had reservations too, but in the end went with it.
I bought a cheap disco light thing from Argos and got her a silver dress and bought silver decorations, plates, silver balloons etc. I also got one of those silver streamer door decorations for them to walk through when they arrived (all a bit 70s and 80s!). Most of this was off Ebay and party websites.
To start with we had cardboard disco glasses and also cardboard microphonesfor them to colour in, with disco stickers (stars, musical notes) and glitter pens. I made the glasses and microphones myself. But with hindsight that was a lot of work and colouring pages might have been easier!
I also got some plastic inflatable guitars (popular with the boys), silver deely boppers, silver and gold dancing wands (the sort gymnasts use), silver wigs.... and they mostly played with all of those and ran about madly, as children do... there was no adult-type posing or anything, I don't think most of them really knew what a disco was anyway!
If I'd had more time/money some face painting would have been good too - you could have had stars, glitter etc, Ziggy Stardust type faces! (you can tell I am a 70s child).
My daughter helped us make up a music cd to have at the party with some children's songs, but also some disco songs (eg Abba, Kylie). We played some dancing games - musical statues, silliest dancer etc.
In the party bags I put a cd copy of all the music we had played at the party and it was very popular (although all the mothers were probably cursing me later!).
So I think you can definitely do a disco party for younger children without it being too adult, it doesn't even have to be too dark, just think glittery stuff and music!