Background: Families of children with developmental delays (DD) prior to the COVID-19 pandemic experienced inequalities in accessing health and social care services. Measures put in place to combat the spread of the coronavirus have potentially exacerbated existing inequalities and have led to additional pressures for these families. Methods: We carried out a cross-sectional online survey of parents of young children with moderate to severe DD and challenging behaviors living in England, UK. We asked about the impact the pandemic has had on their family well-being, receipt of support, and post COVID-19 concerns. Results: Eighty-eight parents who are participants in an ongoing clinical trial (EPICC-ID) reported a broad range of challenges they faced during the pandemic: lack of information specific to children with DD; difficulties following social distancing and isolation rules; disruption or pause of health and social care services; deterioration in parental mental health and regression of the child's skills. Future access to services, negative long-term impact of school closures, parental unemployment were the parents' main post-COVID-19 concerns. Conclusions: Families of children with significant developmental delays fear lasting impact of the pandemic on their own psychological and material wellbeing and on their child's health. These families require urgent help to meet major health and social care needs and should be prioritized within an overall children's mental health strategy. The article includes a "commentary" from parents with lived experience.