I do not think.
I have two reasons.
First, looking at smartphones long time is bad for little children’s eyes.
If you spend many times to watch smartphones, you cannot look at many things.
Second, children should study hard but they may think that I don’t like to study.
If children don’t study, they cannot lean many things.

Does this sound natural?

See a translation

I do not think.
- This is an incomplete sentence. It could be worded as "I don't think children having smartphones is a good idea.
I have two reasons.
First, looking at smartphones long time is bad for little children’s eyes.
- Nearly there with this sentence. I suggest rewording it as "First, looking at smartphones for long periods of time is bad for children's eyes."
If you spend many times to watch smartphones, you cannot look at many things.
- This sentence is missing a few words to complete the thought. I suggest "If you spend lots of time on smartphones, you may miss out on a lot of things in the world."
Second, children should study hard but they may think that I don’t like to study.
- Here you mixed up some word forms by using "I." I is referring to you, the writer. Instead I suggest "Second, children should take their studies seriously, but they will feel less willing to study.
If children don’t study, they cannot lean many things.
- This could use a few more words to sound a bit more natural like "If children don't study then they won't learn many things."

I hope this is helpful! Let me know if you need any more help!