Laughing and frivolity:
Laughing and frivol behavior is forbidden on Tisha B’av. Doing so can lead to physical danger.
The Poskim severely warn against the custom of throwing acorns around in Shul on Tisha B’av and doing so is a great sin. Doing so is a desecration of a Shul, which is akin to the Temple, in addition to the prohibition of frivolity on such a day. Nevertheless, this was the custom in certain Chassidic communities.
May one hold a baby on Tisha B’av?
Some write one is not to hold a baby on Tisha B’av for pleasure purposes, just as is the law of an Avel within the seven days of Aveilus. One may hold a baby for his own benefit, such as to calm him down, feed him or change him
May one play cards or games on Tisha B’av?
One is not to do so, in order not to remove one’s mind from the Aveilus.
 Mishmeres Shalom 40:8
 Based on Rav Pinchas of Koretz in name of Baal Shem Tov. [Mishmeres Shalom ibid]
 Kitzur Shlah; Elya Raba 559:17; Chayeh Adam 135:17; M”B 559:22
 Seemingly this custom was widespread in certain areas; See also Divrei Yisrael 155
 See Sefer Ish Tzaddik p. 24 [memoirs of Rav Aryeh Levin] for a horrific story of how one Tisha B’av a youngster threw an acorn to the beard of an elderly Rabbis, and the elderly Rabbi began crying “The Beis Hamikdash was burnt today”. That night a fire erupted in the village and consumed the house of these youngsters together with them inside. Their ash was brought to burial on the day of Tisha B’av. [Nitei Gavriel 65 footnote 25]
 See Toras Menachem Hisvadyus 1951 2:262 “In Lubavitch they would play with Berelach on Tisha B’av, and it was not exactly against the spirit of the Sages.”; See also Sefer Haminhagim p. 98 [English] for a story regarding a prank that was played on the Ruziner Rebbe on Tisha B’av, and the Ruziner Rebbe’s response “If your children do not observe your Yom Tov, take it away from them”
 Nitei Gavriel 2 p. 653 based on GR”A 554:21
 Yoreh Deah 391:1
 Nitei Gavriel 79:5