The Shulhan Aruch (552:2) rules that one should not eat fish during the Seudat Hamafseket, the last meal before the fast. The reason is that here are some places which refer to the flesh of fish as "meat," which is prohibited. However, the Poskim are lenient regarding sardines or canned tuna, which are not dignified foods worthy to be served on a royal table.
The Poskim also discuss whether someone who is exempt from fasting on Tisha B’av, e.g. a sick person or a minor, is bound to observe the restrictions of the Seudat Hamafseket. Hacham Bension in his Or Lesion (3:28) rules that even though they will not be fasting, they must observe all the Halachot of the Seudat Hamafseket. He cites the Me’am Loez, in Parashat Devarim, that this applies to minors from the age which they can understand the mourning over the Bet Hamikdash, which is usually from age nine.
-In general, one may not eat fish for the Seudat Hamafseket.
-People who are exempt from fasting are still obligated to observe the Halachot of the Seudat Hamafseket, including children form age nine.