Parents boost STEM skills by scaffolding children's attention and discovery during play, but many need support doing so. Using human-centered design (HCD) methods, we created activity kits fostering parents' (a) involvement in and (b) valuing of parent-child play to promote preschoolers' STEM skills. Study 1 documents how HCD methods informed the design of guided activity kits. In initial home visits, we videorecorded six parent-child dyads playing with basic building materials. Play revealed minimal parental STEM scaffolding and talk. Collaborating with 18 families and drawing on prior research, parent interviews, videotaped play sessions, and advisory-board members' expertise, the interdisciplinary research team designed and refined activity kit prototypes. Study 2 was a randomized field test comparing use and evaluation of final guided kits (n = 50) versus basic kits (n = 25) that contained identical building materials and challenges but omitted scaffolding guides. Both groups received kits by mail every other week for 10 weeks. Relative to parents given basic kits, parents given guided kits (a) reported significantly more sustained use of the kits across the 10 weeks, (b) felt more self-efficacy in fostering their child's STEM learning, and (c) judged that their child had achieved greater STEM-skill learning from program use.