The more locutions the kids hear over and over and have access to, the more locutions and idioms and so forth they start to add to their regular vocabulary. To this end, I've put up an idiom wall. Each week, each level chooses an idiom which becomes the Idiom of the Week. The kids, when they use it correctly and in context, get a treat. This means I go through candy very quickly, but it also means that kids are falling all over themselves to use idioms correctly - which means they get committed to memory very quickly. This week, Latin II is practicing excidit mihi and Latin III has nisi fallor. If you use a denarius (or somesuch) rewards system, that would work nicely for this, as would a number of other systems.
Secondarily, every time my kids and I finish a text, we comb back through it to see if we can find phrases and locutions that we like and that are useful in general speech. Out of Genesis, for example, they liked in escam and ad vescendum, as well as the very authoritative factum est ita. We keep this running list on the wall for them to access, and because they've chosen the phrases, they're generally ones that make sense to them and that they're likely to say and use.
These phrases are icing on the cake - the kids aren't tested on them and they aren't on a regular 'vocab list' - but they're up and posted all the time so the kids have access to them and can use them at will. The first part of comprehensible input is always comprehension - if they don't understand, we may as well be talking to a wall. They almost always choose phrases they understand. The second part is that it be compelling - the input can be as comprehensible as we like, but if they're not interested, it won't stick. When they choose the phrases, they're generally ones attractive to the kids, so they stick better and are more useful to them in general.