Maybe Caroline's children don't want the position. Why should it be forced on them if they don't want it. I don't think they were raised (trained) for it so it's probably a daunting prospect to them. They seem happy with their life as it is.
In my opinion it is Albert's responsibility to provide the heir.
I think they were raised to understand the responsibilities. Each has chosen a different aspect of Monaco's heritage. Andrea does a great deal of humanitarian work under the radar. Pierre who is about to or has graduated with a degree in economics and Charlotte is involved in the fashion and art world. All three spent time with their Grandfather and at royal functions through out their childhood. Caroline since her Mother's death has shown a great deal of duty and I'm sure she has raised her kids to feel that sense as well. We don't know what has gone on behind the scenes, but I think in a few years, Andrea and Pierre will be quite ready should they need to step up to the job.