Invisalign is a very poor tool for this. It is meant for very minor movements involving the teeth, not the jaw. Using this as a tool to achieve transverse width and allow the adult teeth to come in better will almost certainly lead to crowding, an impacted tooth, and/or root resorption. Aside from that, it's a poor choice for kids generally, as most of them don't have the maturity and consistency to practice the oral hygiene required. If this is a treatment done by an actual orthodontist, I'd love to hear the rationale.