Hello Robyn, how to keep everybody interest! A big problem here in the Netherlands with both senior sides and youth teams. We have very little teams at the clubs so more expierenced players have to play with newbie's.
Things I have done in the past:
Organise a "Fasttrack" for the newbies with a separate coach.
Focus on individual positions: like a training session for backrow. All the other players are used to work with this unit. Next training session - other position. (I usially do individual on Tue. and Unit / Team on Thu.
Really get organised and involved with representative teams, promote your top players
Wnat takes up a lot of time is very motivational is set up individual goalsetting plan. With this approach you can discuss individual development but also gives you the opportunity to steer the top players towards team goals.
Hope this helps....