She is a good teacher = satisfactory in quality or quantity.
It is good that you are here = right, proper.
He did a good deed = kind, generous.
They have a good name = honorable or worthy.
Her credit is good = financially sound.
He was a good man = morally excellent.
She demonstrated good judgment = sound or valid.
Fresh fruit is good for you = healthy, beneficial.
I have good news = favourable
The meat is still good after three months in the freezer = edible.
I am in good spirits = cheerful, optimistic.
She is a good friend of mine = close or intimate.
I am good at arithmetic = competent or skillful.
It’s a good day for fishing = advantageous.
He has good manners = socially adept.
I wore my good suit to the wedding = best or most dressy.
We stock canned goods = articles of trade.
She will come to no good = end in failure.
I am leaving, this time for good = finally and permanently.
I could go on with examples but I am pretty sure that you get my drift. Telling a child to “be good” is not an effective manner of guiding their behaviour as good has so many meanings, many of them are completely variable, and our interpretations of them are inconsistent, ie. what I think is good might be completely different to what you think is good.
So, instead of telling your child to “be good,” provide them with a specific instruction regarding what you are expecting from them behaviourally, given the situation you are entering. For example,
“When Elsie comes to play we share the playdough with her.”
“We use our walking feet inside.”
It may take a bit more thought and some practise to get into the habit of being more specific about your expectations but I can assure you that it will be a whole lot more effective than telling your child to just “be good.”