i also teach this age group for 30 minutes once a week (I have 15 different classes)
I have found that generally there attention span is about 5-7 minutes. And I have found that using repitition in my activities is great also so I do the same activity every other week as the first time I do it they are just figuring it out and understanding directions and the second time they typically understand the activity better and put the vocab into their memory instead of trying to figure out my directions!!
Things I have found to work:
1. "Feed the Monster": I took an old folgers coffee container and turned it into a monster. Then I hand out my flashcards and say, "oh the monster is hungry, who can feed it?" "It wants...insert vocab word" THEY LOVE IT! Took a while to get them to fully understand (about 3 times doing it but now the get it and get excited when I bring the monster to class!)
2. Can you Find it?: Simple but I do it the first day I introduce new vocab words to a group I make my flashcards with black paper on one side so i turn them with the black paper facing up and say, "Can you find vocab word? and then they get to take turns turning over one card. I then take the card from them show the class and ask "is this the vocab word?" They have fun yelling no and laughing at me for thinking it was
3. Treasure hunt: Turned a tissue box into a treasure box and then before the kids come in the room I hide flashcards throughout. They have to find them, bring them to me and say the word in exchange for a gold coin! THEY LOVE IT!
I do a bit more but these are the kids favorites. My general class plan is: Hello Song, vocab review, game (one of the above and a few more I have), song related to topic, storybook (i have to get them to basically memorize each monthly book per my school's curriculum so the first time I read it to them and then every other time they echo read), goodbye song.
Sometimes I do felt things too...we just are doing facial parts so I made faces out of felt and the kids get to put them together. I ended up making 13 so each kid could have their own and then I'll ask the class, what is this and hold up a mouth. If they'll say the word they get a mouth for their face...lots of work for this one but I'm doing facial parts for two months so I figured it was worth it!
Good Luck. Don't get discouraged and if something doesn't work the first time don't give up on it BUT do it again. They'll never fully understand the first time but by the fifth time you won't have to even explain they'll just get excited!