I have taught languages to students aged 5 to 14. Most of my experience has been in primary schools with one lesson per week for 45-50mins.
I have followed a structure that worked for my students and was easy for me to implement. Basically a 5-10 minute "warm-up", follow by the lesson content, finishing up with a 5-10 minute "cool-down".
In the warm up, I usually have the vocabulary that I am focusing on for that particular lesson. For younger students I might have 6-10 words, middle primary 10-15 and for upper primary 15 words as a minimum. Some of the vocabulary had already been learned previously and it is always important to be aware of students abilities. You don't want to overwhelm a 10 year old student with 18 completely new words as it only sets them up for failure. The vocabulary in each lesson builds on what has been learned the previous year, or earlier in the year.
I ask that classroom teachers have their students sitting in a circle at the beginning of the lesson. I quickly take attendance, and they need to say "Here/ Good morning/ Good afternoon/ etc" in the target language. The reason I take attendance every week is to help me remember names (!) and I find it sets the tone for the lesson. They know that this is how we start a lesson. I then spend a few minutes going through the vocabulary for the lesson. I will say a word in the target language and the students repeat back to me. I also have a chart with the words in both the target language and English for students to refer to. It might sound a little boring for older students but kids are great imitations and you can go through really quickly or even have a play around with shouting or whispering the words. Once we have been through the vocabulary, I mix up the order and alternate between the target language and English.
Then we move to the core of the lesson. For most classes, students have a big scrapbook that is used just for languages, or a display folder can work well. I usually have around 5 different activities that relate to the vocabulary focus for the lesson. I try to mix it up each week and consider all the macro skills involved in language learning (reading, writing, speaking and listening). Games like Bingo, Memory, Snap, Dominoes, etc are fun for students to play with. I like to do at least one written task so there is evidence of their learning. For younger students it could be a simple picture that they draw and label in the target language. For older students it might be some short sentences modelled by the teacher and then they can draw a picture to match. Encourage students to create their own sentences after modelling. Eg: "I like to eat oranges and apples," "I like to eat bananas and strawberries". "I don't like to eat kiwifruit". If students are confident they might be able to do a role play focussed around the topic for that lesson. It all depends on the students ability and it is about taking it just a little further as the year progresses.
I also like to include a cultural task, or even just a bit of general knowledge or trivia for students. Of course it depends on the language you are teaching but as an example in Indonesian the word for grey is abu-abu, while the word for ash is abu. In Indonesian there is no word for 'pink'. They say mere muda, which is light red.
At the end of the lesson, I like to ensure that we are packed up so students are ready for their classroom teacher to return. I finish off with a similar activity that we did at the beginning. It is also a good time for students to share their work, particularly if they have created some of their own sentences or role plays. If there is any time I might play 20 questions where I am thinking of a word that we have learned in that lesson.
I usually change topics every 1-2 weeks, this keeps students interested. I have tried doing a 5-6 week unit on a particular topic, but I found after a few weeks students were getting bored with the content. A 5-6 week unit can be followed throughout the year, mixing it up with other lessons. Of course, it does all depend on the expectations at your school. Sometimes I plan a lesson and I might have to take it over 2 weeks. Other times, students get through things faster than I anticipate. I always make sure I have a basket of "goodies" just in case this happens. Colouring in, extension activities, extra games or even jigsaw puzzles can be great fillers if you need them.
Good luck & happy teaching!