The word "romping" refers to lively and playful behavior, often associated with children or animals. Antonyms for "more romping" could include words that connote a more subdued or serious tone. Some possible antonyms might include "less playful," "more serious," "less energetic," "more restrained," "more subdued," "less exuberant," "more controlled," "less frivolous," and "more somber." Depending on the context, other antonyms might be appropriate as well, such as "more focused," "more intense," "more structured," "more disciplined," and so on.