Hi Erin, I like your travel style, fortunately it is easy to support locals in Tonga. You can buy produce from roadside stalls, use taxis (for many families taxi driving is how they make a living), buy handcraft from the makers at Talamahu Market and Langafonua Handicraft Centre, drink coconut water and local coffee - have a look at madeintonga.com, that will give you an idea of all the products and food you can buy that is Tongan made.
The article 'Coconut Industry Empowers Tongan Farmers' is an interesting read and mentions Tongan made Fiesta Ice Cream and their topsy bars. Having dinner at the Áhopanilolo Technical Institute will also support hospitality students in training.
Also, ask the Heilala staff for ideas as well as the staff at the Visitor Bureau on Vuna Road, they should have more suggestions.
Happy travels in Tonga, I hope you both have an enjoyable trip.