Rwanda is a very safe and reasonably well-developed country, so you needn't worry about travel with children here. However, the genocide remains prominent at many sights in Rwanda and could be confusing and traumatic for younger children who may struggle to understand the events of 1994.
Otherwise, nappies and baby formula are widely available, children are welcome in restaurants, hotels and resorts, and most will love the chance to see wild animals in the national parks.
For older kids, wildlife-watching in Akagera National Park and Nyungwe Forest National Park is a sure-fire hit. Note that kids must be over 15 for gorilla tracking. Gisenyi (Rubavu) and Kibuye (Karongi) have excellent resorts that are suitable for families.