Personally I think too much is made out of what game they go to. Practice is where kids truly develop. I understand the need for game reps, for sure, but that's not always my main concern. We tell the "tweener" kids they're going to play where we can get them 4 ABs a game (based on talent of course, which these kids have since they're the "tweeners"). If that's a varsity game, so be it. If it's JV, same story. We have some weaker teams in our district, so that impacts our decisions from week to week as well.
But all of our "good JV kids" practice with Varsity every day. They rep everything just like the varsity kids, defense baserunning, signs, dirt ball reads, everything. Hell, we only hit 18 kids in BP (not always, but our most often used BP format), and we have some of those JV kids hitting in groups and some seniors not hitting. Talent decides reps in practice, too.
Later in the year I'm more inclined to bring them to Varsity games even if I'm not sure they'll cross the lines, just to get used to the atmosphere and routine. They'll likely be with us for the playoffs so I want it to feel as normal as possible at that time.