Broadening participation in early science, technology, engineering and math learning outside of school is important for families experiencing poverty. We evaluated variations of the Teaching Together STEM pre-kindergarten program for increasing parent involvement in STEM learning. This informal STEM, family engagement program was offered in 20 schools where 92% of students received free/reduced lunch. The core treatment included a series of family education workshops, text messages, and family museum passes. The workshops were delivered at school sites by museum outreach educators. (...) We randomly assigned schools to business-as-usual control or one of three additive treatment groups. Using an additive treatment design, we provided the core program in Treatment A, we added take-home STEM materials in Treatment B, and added materials + parent monetary rewards in Treatment C. The primary outcome was parent involvement in STEM. There were no significant impacts of any treatment on parent involvement; however, the groups that added take-home materials had larger effect sizes on parent involvement at posttest and later, kindergarten follow-up. Adding parent monetary rewards only produced short-term improvements in parent involvement that faded at follow-up. We discuss implications for other community-sponsored family engagement programs focused on informal STEM learning, including considering characteristics of families who were more versus less likely to attend. These null findings suggest that alternatives to in-person family education workshops should be considered when parents are experiencing poverty and have competing demands on their time. (shrink)