Minibeast is not a proper science word, but a teaching word. It is simply a catch-all word that is easy to say and to remember, and refers to any small animal that you might find in a garden, school yard, pond, woodland or park.
It is easier to collect and study very small creatures than big ones, and it is often something that can be studied outside in spring and summer.
Studying small creatures is a good way of studying where animals live (habitats), and how they live as parts of food chains.
Quite often a whole variety of small animals are studied at the same time under the ,minibeast heading, so a study of spiders and butterflies might be done at the same time as snails, slugs, worms, tadpoles and small pond fish.