Verb (type 1): To play.
Leikkiä appears in our example in the partitive plural of its active present participle form. (See leikkiä conjugated here.) In its participle form it functions as a verbal adjective meaning ”playing.” Here it is used to modify the word lapsia (”children”), the partitive plural of lapsi (”child”).
Note that the present participle of a verb is subject to declension. Leikkivä is the basic form of the active present participle, which is then declined as shown here.
The phrase ”leikkiviä lapsia” means ”playing children.”
Let’s break down the other phrase on the sign, ”Aja varovasti,” which means ”drive carefully.”
Aja is the second person singular imperative form of ajaa, which means ”to drive.”
Varovasti is an adverb meaning ”carefully” or ”cautiously.”