fluency or proficiency

[Quaerereverum]

Hello,

My sentence is about language acquisition.The reading claims that kids learn faster, but the lecturer refutes it. Can I use the word ''fluency'' or should I use ''proficiency''?

The professor points out that children use simple sentences and they are at the low level of fluency/ proficiency.

Thank you in advance.

[IELTSbuddy]

I think it will be proficiency. Proficiency is about ability, so that would refer to using a 'simple sentence' as they are unable to use more complex structures.
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