Background: Transverse crossbite is one of the most frequent malocclusions in orthodontics and requires early intervention. Rapid maxillary expansion (RME) is a well-established and effective technique for the correction of maxillary transverse deficiency; however, several studies have also investigated maxillary expansion performed with orthodontic aligners in the deciduous dentition. This study aimed to propose a new, clear aligner-based clinical protocol, applicable to very young children with complete deciduous dentition, to enable early correction of posterior crossbite. This protocol has been termed the Baby First Expansion Protocol. Cases: This case series describes three pediatric patients with deciduous dentition and maxillary contraction treated with orthodontic aligners following the Baby First Expansion Protocol. The first case involved a 5-year-old girl with deciduous dentition and functional unilateral posterior crossbite associated with a Class II malocclusion. The second case involved a 5-year-old girl with deciduous dentition and a Class I malocclusion associated with a rightward functional mandibular shift due to unilateral posterior crossbite. The third case involved a 5-year-and-2-month-old boy with deciduous dentition, a Class III malocclusion, a functional unilateral posterior crossbite, and a leftward mandibular shift. In all cases, pre- and post-treatment intraoral scans were acquired using the iTero Lumina scanner, and stereolithography (STL) files were uploaded to OrthoCAD software (version 5.9.1.50) to analyse changes in the maxillary arch following expansion. Conclusions: This case series highlights the importance of early diagnosis and interceptive treatment of functional unilateral posterior crossbite in pediatric patients with deciduous dentition. The Baby First Expansion Protocol appears to be an effective and well-accepted treatment option for the correction of functional unilateral posterior crossbite in very young children, although it remains a compliance-dependent approach. Further prospective studies with long-term follow-up are required to confirm the stability of the results and to better define the clinical indications of this protocol.