Child language acquisition might seem tricky but if you've got the basics of the linguistic framework - lexis, semantics, syntax, morphology, phonology and pragmatics (you know it really) - and some idea about the stages and theories, you'll be laughing. Oh, and make sure you have plenty of examples of what children say.
Here's some stuff from the blog 2 years ago about preparing for this topic. The same things apply, but it's probably worth noting that there's been a more recent focus on data-based questions; these are ones that give you some child language data and ask you to comment on what it tells you about "the nature of child language acquisition". It's therefore a good idea to look carefully at the data and think about what it might reveal about issues like imitation, interaction, cognitive and . Check the feedback sheet here for some ideas about the data from a year or two back. Also, check the feedback sheets that I put together for 2 other questions here and here if you want some more detail.
There's also more feedback and some data examples in this powerpoint.