How do you translate 养得汞儿性圆，等待他铅儿花现 in a text about 睡仙气功?
the verse quoted is, well, different from the original text, which is in 道藏輯要 三丰全集 玄要篇下：
蟄龍吟

睡神仙・睡神仙・石根高臥忘其年・三光沉淪性自圓・氣氣歸玄竅・息息任天然・莫散亂・須安恬・温養得汞性兒圓・等待他鉛花兒現・無走失・有防閑・眞火候・運中間・行七返・不艱難・煉九還・何嗟嘆・靜觀龍虎戰場戰・暗把陰陽顚倒顚・人言我是矇矓漢・我卻眠兮眠未眠・學就了眞臥禪・養成了眞胎元・臥龍一起便昇天・此蟄法・是誰傳・曲肱而枕自尼山・樂在其中無人諳・五龍飛躍出深潭・天將此法傳圖南・圖南一派儔能繼・邋遢道人張丰仙


in taoism, the lead (鉛) - mercury (汞), dragon (龍) - tiger (虎) are metaphor. depends on the alchemy (external or internal); and sects, these terms might be explained diversely.

Is this translation on the right track? I'm particularly not unsure whether “儿” in this phrase is supposed to be a diminutive suffix or not

on the right track? i believe not. “兒”, yes, a diminutive suffix.
try this page, in english, not too peculiar 
https://deeptrancenow.com/exc_taoist_language.htm