Background/Objectives: In the UK, significant and rising numbers of children arrive in schools with marked deficits in key skills such as oral language. This rise has been further negatively impacted by the COVID-19 pandemic. Given this, the foundation phase of primary school education is a necessary environment for targeting language deficits. There is evidence to suggest that teaching assistant (TA)-led interventions can be effective when adequate training and support are provided. This study explored the feasibility of providing a brief, online dialogic book-sharing training to TAs, and whether this training would be effective in upskilling TAs and enable them to improve the language outcomes of children aged 3–7 years in a school context. Methods: North Wales primary schools were invited to nominate TAs for the two half-day training sessions. Five schools responded, and eleven TA–child dyads participated. Data were collected on recruitment, training acceptability and baseline, and post-training measures from TAs and children (2–3 weeks after the final training session) and 4–6 weeks after the first follow-up. Measures of TA competence and behavior were collected, along with measures of child language and behavior. Results: Schools and TAs were recruited; TAs reported positively to the training, and the results showed small to large effect size benefits on all TA skills and child expressive language with significant positive post-training effects on TAs’ use of reflections and child language abilities. However, these effects were somewhat reduced at follow-up. Conclusions: Overall, the results of this feasibility study provide positive evidence for this training as an accessible way for schools to strengthen their prevention infrastructures by professionalizing a growing, but relatively untrained, group within the school workforce.