Noun
ひざこぞう
自転車で転んで、膝小僧が痛い。
I fell off my bike and my kneecap hurts.
最近、右の膝小僧にタトゥーを入れたんです。
I recently got a tattoo on my right kneecap.
膝小僧の調子はどうですか？
How's your kneecap?
In your knee there is a small priest named Kneecap.
This is a really common, cute way of talking about your kneecap. It's usually used by kids and adults alike, and it's more common in spoken Japanese than 膝頭. Kind of like how we use "kneecap" instead of naming the "patella" bone all the time.
This word uses the kun'yomi readings for 膝小 plus the on'yomi reading for 僧. You've learned them all already, so you should be able to read this as long as you remember them all.