We describe two experiments on signaling and detecting uncertainty in audiovisual speech by adults and children. In the first study, utterances from adult speakers and child speakers (aged 7-8) were elicited and annotated with a set of six audiovisual features. It was found that when adult speakers were uncertain they were more likely to produce fillers, delays, high intonation, eyebrow movements, and "funny faces." The basic picture for the child speakers was somewhat similar, in that the presence of certain audiovisual cues correlated with uncertainty, but the differences were relatively small and less often significant. In the second study both adult and child judges watched responses from adult and child speakers selected from the first study to find out whether they were able to correctly estimate a speakers' level of uncertainty. It was found that both child and adult judges gave more accurate scores for answers from adult speakers than from child speakers and that child judges overall provided less accurate scores than adult judges. (Contains 10 tables and 3 figures.)