ではあるまいし / じゃあるまいし

ではあるまいし brushes an idea aside and uses that as a reason: "it's not as if…, so…". The first half names something that is plainly not the case, and the second half says what follows – usually a scolding, advice or reassurance. It goes straight after a noun or a な-adjective; a verb needs わけ or の first, or ん in the spoken form.

あるまい is the literary "probably is not", and the final し gives the reason.

ではあるまいし or ではあるまいか

ではあるまいか ends in か and does the opposite: it puts an idea forward as probably true. ではあるまいし rejects the idea.

Without し, ではあるまい is a plain guess, "probably is not", and does not lead into a second half.

• × 子供ではあるまい、自分でやりなさい → 子供ではあるまいし、自分でやりなさい

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じゃあるまいし is the spoken form and the one you'll hear. It sounds like a retort or a light telling-off, often from someone older. ではあるまいし is its written form. The everyday, softer way to say the same thing is じゃないんだから.