Your child's fish may be located in different tanks. To access additional Aquarium tanks on the mobile version of ABCmouse:
1) Tap on the yellow fish on the top-right corner
2) Tap on the fishbowl with 3 orange fish located to the right of the fish tank name
3) Tap on the left or right arrows to navigate between available fish tanks
From there, you should be able to locate any additional fish and move them from tank to tank.