They will not necessarily extrapolate from the event. Even if they do kids are more robust than you'd give them credit for. I have a year seven form group who were eleven years old when I inherited them. The amount of dysfunction in many of their families render a single parting of the ways as insignificant and none of them appear to be emotionally troubled by the rifts, remarriages and turbulence in their lives. The most troubled kids in my form are the ones whose families stick together but have a high degree of animosity between family members.
I'd gently break the news to them. I'd not make any links to your own family unit or even attempt to reassure the kids that you and your partner will not do the same as that's likely to sow the seeds of doubt. Answer and questions that they raise and leave it at that.