This paper examined to what extent causal explanation speaking tasks (CESTs) are cognitively appropriate for assessing young language learners' (YLLs) L2 speaking. Ninety-six YLLs (48 from Grade 4 and 6 each) in China performed two CESTs in both L1 (Chinese) and L2 (English). They also completed receptive and productive L2 vocabulary size tests. We examined how their CEST performance scores, choice of causal antecedents, and speech utterances were related with language modes of the tasks (L1 vs. L2), grade levels, and L2 vocabulary sizes. L2 CEST performance scores were found to have significant positive correlations with L2 productive vocabulary size. CESTs were found to be generally cognitively appropriate for YLLs because their high scores in L1 performance indicated that performing CESTs is within their L1 capacity. By examining causal connectives used by YLLs, we found that learners from both age groups had cognitive ability sufficient to verbalise causality. Yet YLLs' cognitive ability to interpret and verbalise mental states is still developing and reasoning between causal antecedents that have competing causal relationship with the final state can be cognitively challenging. We discussed the findings with reference to the design of cognitively appropriate CESTs that can assess both language and thinking skills.