Enemies such as The Tomb Guardian will summon skeletons during battle. Do these minions give XP when they are killed?
Yes, Skeletons summoned by the Tomb Guardian give both XP and can also drop items and gold.
It's a nice bonus, to be sure, but given the slow rate it takes for Tomb Guardians to summon more of their friends, you'll have better gold and XP rates if you just kill them and press on.
As far as I'm aware (and in my own experience), all summoned monsters give xp. Most summoned monsters can drop loot too, with the exception of boss adds (or if they do, it gets added to the boss' loot).