There's an interesting -- understandable but very wrong -- viewpoint that the right way to form sentences that a second-language speaker can understand is to talk in the way you might talk to a toddler.
Reality is pretty much the opposite. A toddler has perfect grasp of their language's grammar and is likely to know common words, less likely to know uncommon words. For best results, you'd communicate with a toddler using complex sentences and "easy" vocabulary.
A foreign learner has a much shakier grasp of the grammar and may have a pretty spotty vocabulary. But they're also likely to have a dictionary. And uncommon words are much easier to understand, given a dictionary, than common words are. The fact that they're uncommon means they don't have the wide variety of meanings and usages[1] that common words do. For best results, you'd communicate with a foreigner using simple sentences and "difficult" vocabulary.
[1] For reference, here are the ABC dictionary glosses for the very common Mandarin word 做:
1. make; produce
2. cook; prepare (food)
3. do; act; engage in
4. be; become
5. write; compose
6. celebrate
7. be used as
8. form/contract a relationship
9. pretend; feign
10. (slang) make love
11. play tricks to punish somebody