Hello, I would say that the level of English of the diagnosed children can be studied using quantitative methods, i.e. stylometry, vocabulary richness. Thať could be a fruitful way of assessing their progress.

For me, I worked on the L2 acquisition for autistic children in the light of Chomsky's innateness versus the interactionist theory; verbosity vs selective mutism is another aspect, or the language acquisition via different, sometimes unconventional modalities (written format), anyway it is a very rich and dense topic, good luck.

You need to do a literature review to find this gap. Good luck!

In order for you to find the gap, you can do a systematic literature review (SLR).