"their money was not earned easily" may not be a commonly used set phrase, but there is absolutely nothing wrong with it.
If you were writing very formally, however, the more common "was hard earned" might be better.
ell.stackexchange.com/users/2748/nxx
There are two main ideas that you want to convey: your parents were not well paid (or underpaid ), and their money was hard earned .
To rephrase your sentence to be more idiomatic, here is one possible way: When I was young, my parents were not well paid.
I knew that their money was hard earned, so I rarely ask them ...
ell.stackexchange.com/users/3281/Damkerng T.
"Their money was not earned easily" is not a commonly-used phrase, but it is perfectly clear what you mean and I think it is well said.
Yes, you could say, "They had a low income", but that's not as colorful.
ell.stackexchange.com/users/803/Jay
As OP has discovered, money isn't earned easy (or easily ) is not a particularly common turn of phrase.
There are no written instances of either of those exact forms in Google Books, but there are hundreds of instances of money doesn't come easy (and easily ).
Far more common (Google Books claims about 11,200 results ) is... money was hard to come by ...
which I think focusses more on the fact that there wasn't much money, even if you worked hard .
ell.stackexchange.com/users/126/FumbleFingers Reinstate Monica
Just to point out, you already provided a better idiom: "I knew that their wages were low .
So I rarely asked them for an allowance to buy candies or toys."
However, some others may be more popular:
they didn't make much money, we were poor, we didn't have much money, they didn't have much money, they were underpaid
I suppose any of those could work, depending on what you want to mean and the "voice" you want to use.
ell.stackexchange.com/users/3796/CoolHandLouis
Suppose that when I was a child, my parents' wages were low. Is it good to write:   I knew that their money was not earned easily. So I rarely ask them for allowances to buy candies or toys.   In particular, is it good to write  their money was not earned easily ? I found  money was not earned easily  appears very rarely in Google search.
"Their money was not earned easily."
ell.stackexchange.com
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ell.stackexchange.com/questions/15569
ell.stackexchange.com/users/499/Tim
Their money was not earned easily is not a commonly-used phrase, but it is perfectly clear what you mean and I think it is well said. Alternative suggestions include: they didn't make much money, we were poor, we didn't have much money, they didn't have much money, or they were underpaid. This is not common but not incorrect either. Didn't make much money, were poor are alternative renderings.
This is not common but not incorrect either. Didn't make much money, were poor are alternative renderings.