I would suggest going to a fish store and seeing what you like. Live bearers can be supercute, tetras come in all shapes, sizes and colours. Danios are very active. Corys are great to watch when in large enough numbers to see their natural school behaviour. My concern with 2 year olds would be getting something big enough and colourful enough to hold their attention, so possibly small schooling wouldnt be a good choice. Maybe a group of mollys would be good. They get to a decent size, and can be very sociable. A 29 gallon would accommodate a decent group of fully grown mollys.
As for decorations and tank set up, i would really consider a low tech planted tank with some driftwood and rocks. Java ferns, java moss, anubias, and amazon swords really dont take any effort apart from turning a light on for 8ish hours a day, and the benefits in terms of keeping water parameters under control and having a natural looking aquascape will make it worthwhile.