外国人の子供に説明する際は、文化的にあまり素直でなかったり、理由を知りたがるので、なぜダメなのかの理由を具体的に説明した方が良いでしょう。
その場合は、
・Make sure you don't put the glasses facing down because it could create scratches
on the lens and make the glasses harder to see.
（上記翻訳）
メガネを下に向けて置くと、レンズ傷つけてメガネを見難くしちゃうから、下向けに置かないようにね。
ここまでしっかりと説明すれば、子供であってもしっかりと理解してくれるでしょう。
Don't put your glasses on the lenses side (otherwise they might/could get scratches).
前置詞”On”には何かを支えるイメージがあります。この場合は、レンズ側が机などと接してメガネの重さを支えているイメージです。同じ用例で、警官が容疑者を確保する際に”Get on your knees!” と言うことがありますが、この場合も両膝が地面に接して体重を支えるイメージから”On"が使われます。
Don't put the glasses down on the lenses because they will get scratched up.
Set your glasses down on the frames so the lenses don't get scratched up.
Please set your glasses on the frames, not the lenses.
Don't put the glasses down on the lenses because they will get scratched up.
Set your glasses down on the frames so the lenses don't get scratched up.
Please set your glasses on the frames, not the lenses.
Make sure you set your glasses on the frames instead of the lenses.
Please don't put your glasses down on the lenses. Set them on the frames instead.
That's not good to set your glasses on the lenses. They will get scratches on them. Set them down on their frames.
You might want to set your glasses on their frames instead of the lenses so they don't get scratched up.
Let's put your glasses on their frames instead of the lenses so they stay nice.
Don't put the glasses down on the lenses because they will get scratched up.
（傷がつくから眼鏡はレンズを下にして置いてはいけません）
Set your glasses down on the frames so the lenses don't get scratched up.
（眼鏡はレンズが傷つかないようにフレームを下にして置きなさい）
Please set your glasses on the frames, not the lenses.
（眼鏡はレンズではなくフレームを下にして置いてね）
Make sure you set your glasses on the frames instead of the lenses.
（眼鏡はレンズではなくフレームを下にして置くようにしなさい）
Please don't put your glasses down on the lenses. Set them on the frames instead.
（眼鏡はレンズを下にして置かないでね。フレームを下にして置くの）
That's not good to set your glasses on the lenses. They will get scratches on them. Set them down on their frames.
（眼鏡をレンズを下にして置くのは良くないよ。傷がつく。フレームを下に置きなさい）
You might want to set your glasses on their frames instead of the lenses so they don't get scratched up.
（傷がつかないように眼鏡はレンズではなくフレームを下に置いた方がいいかもしれない）
Let's put your glasses on their frames instead of the lenses so they stay nice.
（眼鏡は傷がつかないように、レンズではなくフレームを下にして置くようにしよう）
Don't put your glasses down like that you will scratch the lenses
Don't put them down with the lenses facing down
The glass bit in the glasses is called the 'lens' or 'lenses' as there is two in a pair
If glass gets rubbed it 'scratches' and then it is not as clear as it was if something 'faces down' it is against or facing downwards
You'll scratch the glass if you put them down like that
Don't do that! You will scratch the glass!
To your child. He/she put his/her glasses on the table with the lenses facing down. You want to tell him/her not to put his/her glasses with the lenses facing down, because they might get scratches.
The first example is a simple explanation. The second example is a harsh rebuke.
If you put your glasses on the table with the lenses facing down, they will get scratched and you won't be able to see through them.
Don't put your glasses on the table with the lenses facing down because they will get scratched and you won't be able to use them.
Always put your glasses on the table or any other surface using the frame, the same way as you wear them on your face.
Children always have to be told to do the right things because that's how they learn. They learn through making mistakes. So, normally, a child won't know beforehand that putting his/her glasses down on a particular surface, like on top of a table, with the lenses facing down will damage his/her glasses by getting the lenses scratched. So, you as a parent has to educate him/her to be doing the right thing at all times, that is to put his/her glasses on a particular surface using the frame and not the lenses.
So, you may say to him/her:
If you put your glasses on the table with the lenses facing down, they will get scratched and you won't be able to see through them.
or
Don't put your glasses on the table with the lenses facing down because they will get scratched and you won't be able to use them.
or
Always put your glasses on the table or any other surface using the frame, the same way as you wear them on your face.
子どもには何が正しくて何が正しくないのかを教えてやらないといけません。子どもは失敗をすることで学びます。おそらく、レンズを下にして眼鏡を置いてはならないことを子どもは知らないでしょう。ですから、親として子どもにフレームが下に来るように眼鏡を置かないといけないと教えてやらないといけません。
次のように言えます。
If you put your glasses on the table with the lenses facing down, they will get scratched and you won't be able to see through them.
（テーブルにレンズを下にして眼鏡を置くと、傷が付いて見えなくなってしまう）
Don't put your glasses on the table with the lenses facing down because they will get scratched and you won't be able to use them.
（テーブルにレンズを下にして眼鏡を置いては駄目、傷が付いて使えなくなってしまう）
Always put your glasses on the table or any other surface using the frame, the same way as you wear them on your face.
（眼鏡をテーブルに置くときはフレームを下にしないと駄目、眼鏡をかけるときと同じようにね）
"Don't lay your glasses on the table lens down" and "Don't put your glasses on the table face down"
These expressions state that you should not put/lay your glasses onto a surface facing down/lens down.
face up or face down are used to describe a part of something, often the face up is the more important piece of the equipment, in this case the glasses, the face is the lens which is the glass itself.
"Don't lay your glasses on the table lens down"（レンズを下にして眼鏡を置いては駄目）
"Don't put your glasses on the table face down"（眼鏡は下向きにして置いては駄目）
これらの表現は「眼鏡は下向きにして置いてはいけない」と伝えています。
'face up' または 'face down' は物の部分を表します。'face up' はその装置においてより重要な部分であることが多いです。
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