The first five years of life are critical for optimal growth, health, and cognitive development. Adverse childhood experiences, including experiencing homelessness, can be a risk factor for multiple health issues and developmental challenges. There is a dearth of data collected with and by families with children under age five living in temporary accommodation due to experiencing homelessness (U5TA) describing indoor environmental barriers that prevent U5TA from achieving and maintaining optimal health. The aim of this study was to address this current gap using a citizen science approach. Fifteen participants, who were mothers of U5TA living in a deprived area of London, and the lead researcher collected data in late 2019/early 2020 using: (I) a housing survey conducted via a mobile app; (II) house visits; and (III) collaborative meetings. Data were analyzed using thematic analysis. Key themes included: overcrowding/shared facilities, dampness/mold growth, poor/inadequate kitchen/toilet facilities, infestations/vermin, structural problems/disrepair, unsafe electrics, excessively cold temperatures, and unsafe surfaces that risk causing trips/falls, with all participants experiencing multiple concurrent indoor environmental barriers. The citizen science approach was successfully used to collect meaningful data demonstrating the need for child-centered housing policies meeting the needs of current and future generations of families living in TA.