I have had people ask me how to do a French-only lesson with kids as young as JK. What I have found works best is to do activities that are very action-based where you can say the French and it is obvious what it means based on what's happening. For example, I play a lot of games with cards and my current card set for the JK class is squares of different colours. We did a few lesson-type stuff to learn the colours, and now we are reviewing using the cards. Here was my script for day 1 with the cards:
Me: Oh, regarde! Ja'i une surprise [they already know the word surprise from other things]
[I show the cards, they ooh and ahh]
Me: C'est une carte. C'est une carte orange [show it] C'est une carte. C'est une carte bleue. [show it]. Carte. Tout le monde dit: carte. [they say it]
Me: [show orange card] Carte bleu? [they respond 'non' and some shout 'orange' already. Others will wait for next prompt] Non, pas une carte bleue. C'est une carte orange. [point to orange part] Orange. Orange. [they repeat. Now they understand that orange refers to the colour and carte refers to the card that it's on]
Me: [repeat with a few other colours]
Me: Maintenant, j'ai un jeu [they know the world 'jeu' already] Regarde la carte. C'est une carte rouge? Oui or non? [one child at least will answer, usually more than one] Oui, c'est une carte rouge. [hold up another card] C'est une carte jaune? [again, answers from them]
Me: [hold up a card and say nothing but look at them expectantly. A child or two will inevitably shout out the colour.]
Me: Oh! Bravo! Fantastique [etc.] [Give card to child with large, emphatic motion] Carte pour toi.
Me: [repeat until they get one wrong or do not guess. Place card ina second pile away from the kids.] Oh! Carte pour moi!
Continue until all the cards are gone. Then pick up one of the piles, and start counting them. By 'deux' they will be joining in on the counting. If you win (unlikely; I never play the game with them until they are ready to win it) then say 'je gagne!' with large, exaggerated happiness. If they win, say 'la classe [or tout le monde] gagne' with same. And then make a sad face, point to yourself and say 'je ne gagne pas!'
I have started introducing some variations, a few weeks into the game. When I do not win, I say 'Je ne gagne pas, je pleure' and pretend to cry. They think this is hilarious. I also have my puppet scootch over to sit with them before we start playing. He prefers to be on their team because I never win :) Sometimes, he will waver and go back and forth, pondering who he wants to be with. The children will prompt him with 'avec moi!' or 'avec la classe!' with great excitement. Madame not winning the jeu is the most hilarious thing ever. The other day, I forgot to cry, and they prompted me with 'pleure, maintenant!' All in French, JK or not :)