Combine the sushi rice and brown rice in a saucepan with 3 1/2 cups of cold water. Bring to the boil then reduce heat and simmer for approximately 12 minutes or until water is fully absorbed. When the rice is cooked, stir through the soy sauce and vinegar and spread it out on a baking tray to cool down.
Meanwhile, set up a sushi rolling station by laying out a clean tea towel, a bowl of water, nori sheets, cucumber, and carrot at hand.
Meanwhile, scoop out the avocado flesh and mash it through the shredded chicken.
Lay a nori sheet on the tea towel, and top it with a layer of rice approximately 1/2 cm thick, leaving a 3cm strip at the end furthest from you.
Layout cucumber strips and carrot on top of the rice at the nearest end of the nori, along with 3 tbsp of the chicken mixture in a line before lifting the tea towel and rolling the sushi from the filling side down to the far side, pressing the roll firmly to hold the filling in place. Wet the exposed edge of the nori with a little water so that it sticks the sushi roll together. Repeat to make six long sushi rolls. Slice sushi rolls into bite-sized pieces.
For safety please ensure adults are present when knives and stove are in use. To poach the chicken, place the chicken breast in a pot of boiling water for 10 minutes and allow to rest for at least 5 minutes before dicing. If needed, you can substitute rice wine vinegar for apple cider or white wine vinegar, and sushi rice for arborio.