Is it permissible for an uncle to visit, socialize with, and playfully interact with the daughters of his deceased brother in the presence of their mother?

Visiting the family of a deceased paternal uncle is a good thing if it adheres to the legitimate Islamic guidelines, as the paternal uncle's wife and his daughters are non-mahram to you (unless the wife is a maternal aunt, for example). They must observe proper covering and hijab when you enter upon them, and you must be cautious not to be in seclusion with any of them.

If the paternal cousin is an adult, or a boy who has reached an age where modesty is observed around him, then seclusion is negated by his presence.

Visiting the family and entering upon them is permissible as long as temptation is not feared. Playfulness and joking with young girls are permissible, but not with older girls, as that is a means to temptation.