Mustard, Japanese Mustard, Karashi
noun
The spicy child is the child of spicy. So, not as spicy as mamma spicy, but still a bit spicy. This child spicy is mustard. Not too spicy, but not too mild. 辛子 generally refers to Japanese mustard, which tends to be browner and stronger than マスタード (Western mustard). Sometimes, especially as a prefix, it can also indicate spiciness from red pepper, like 辛子マヨネーズ (spicy mayo).
からし
The reading is 辛い (からい) and the on'yomi reading for 子 combined together. It's one of those weird words that mixes the on'yomi and kun'yomi readings.
やっぱりオデンには辛子でしょ。
When it comes to oden, Japanese mustard is a must, isn’t it?
辛子、チューブのやつしかないけどいい？
I only have Japanese mustard in tube form, but is that okay?
辛子もマスタードも、同じカラシナのタネから作られます。
Both Japanese mustard and Western mustard are made from the seeds of the same plant, mustard greens.
辛子に使われるのはオリエンタルマスタードというカラシナのタネ、マスタードに使われるのはイエローマスタードやブラウンマスタードというカラシナのタネです。
Seeds from a type of mustard green called "Oriental mustard" are used for Japanese mustard, while yellow mustard seeds or brown mustard seeds are used for regular mustard.