Co-designing with children is a recent challenge to the development of new products. Children and designers can express ideas through rough drawings, also called "sketches", but, while drawings are common activities, children can also display lack of confidence or engagement during this task. To motivate children before creating sketches, we considered the administration of group games. This paper tested the effects of different group games on participants' impressions and outcomes in subsequent design sketch tasks. Two studies were performed. In study 1, higher game difficulty was associated with negative impressions and less detailed drawings. By controlling difficulty, in study 2 we found that games positively affected children's confidence with their sketches, which indicated to affect design submissions. Results indicated an immediate effect of games on children's design sketch impressions and outcomes. If negative associations are properly controlled, games can be viable motivators for co-design tasks with children.