Idea of getting a tooth cavity filled is to keep that tooth in place till permanent tooth takes it place. A tooth with caries if neglected will eventually need an extraction prematurely and a space maintainer. Also it can cause caries in adjacent teeth and underlying tooth bud of permanent tooth.
Tooth fillings for kids are special fillings with high fluoride content that help in preventing caries progression. These fillings are also available in different colors if your child wants to flaunt fillings