Good questions earn descriptive answers, so that's what I'm gonna give you! First of all, Childe deserves every type of body jewellery there is but my favourite might be tongue piercing! Childe's outfit is the perfect blend of work-appropriate (grey-scale uniform) and playful (tummy peak and red scarf), and that's why a tongue piercing suits him perfectly - a simple metallic ball that you wouldn't notice unless you're looking for it. A navel piercing in a similar style fits nicely too, only noticeable when his jacket moves just enough to expose it.

Of course, I'm a sucker for marking Childe's body so I would make him get even more piercings! The order of which got the piercings would probably be something like ear>navel>tongue>nipple>dick. His first ear piercing he made himself with a needle at the age of 14. From time to time he adds another ear piercing until he stops at 11 total (hehe), 6 on his left ear and 5 on his right. Except for the ear piercings I don't imagine him wearing any other very visible piercings like lip or nose ones.

Oh, I just realized the question was about body jewellery but I went wild with just piercings. Other body jewellery at his ever-growing jewellery case would be a variety of thigh straps, chokers, ankle bracelets and rings. Though I say all that I don't imagine him wearing much jewellery unless someone else dresses him up, at least in the canon universe he seems very practical and not too bothered by appearances. Luckily we know of someone who would make sure to spoil him at every opportunity. (ꈍᴗꈍ)