I play classical music during art lessons and am having trouble finding cheerful pieces which have an even mood throughout with no surprising volume changes. Any suggestions please?

beethovan's wig ... it's classical music but there are words added to them to make it lively~!

Fur Elise?

After a minor detour to suggest Mozart clarinet quintet (perhaps) I thought you might consider instrumental music with instruments noted for their lack of dynamics, eg harpsichord, lute (& by extension much guitar music) which admittedly narrows your musical eras a bit. Or recorder ensembles.
Also the Brandenburg Concertos (if I remember correctly) don't go in for extreme dynamics or shifts of tempi.