First of all, pick a theme: seasons, special events, festivities, national and international holidays, fairy tales... Anything can be turned into an ESL fun activity!
How long should it be? An hour and a half is ideal to fit an introduction to the theme, a craft, a game and a snack.
Ages can range from 3 up to 12 years old, according to the theme and the craft you are planning to do: if your group is large and you have an age gap, you should consider splitting the group and work on different crafts and games in each age group. Also, the language content is going to be different and more challenging for older kids.
Labs have to be creative and entertaining, aimed at the repetition of key words used in different sentences throughout the whole activity. It doesn't have to be a big range of words, it mostly depends on the group age.
Here is an example: "Be my Valentine" has been one of our latest labs, in which the words we wanted children aged 3-6 to focus on, were heart, love, pink, red and card. There were many other St. Valentine related words in this lesson but the most repeated words were the ones above.
Children are very intuitive and they usually figure out their meaning by themselves, just by placing them in several sentences, you'll be surprised!
Planning a good lab takes some research, some patience and it can be time-consuming, but you can find some great ideas on Pinterest and on Youtube, just to name a couple of apps: all you have to do is putting together - and timing - the different activities you have chosen for your lab.
Before launching yourself into a whole set of labs, you could test this activity in your area with a one-time event such as Halloween or Christmas: you can find thousands of cute ideas for any age group.
You can advertise through social medias, newsletter, whatsapp groups, bulletin boards, nearby schools and local newspapers. I suggest you don't do it too far in advance or too close to the date, 10 days leaves parents plenty of time to get organized. Give a time limit for bookings, generally 3 days before the actual date. This way you will have enough time to prepare the material for the craft, print flashcards and games and buy some treats if you are planning to include a snack.