This is of course a very complicated question. It depends on many factors, e.g.:
- What is the overall goal of the class? (to help them succeed in careers in the future?)
- Are the students illiterate in their L1s?
- What is most important for these children - reading. writing, speaking, or listening? Or a balance of all of these?
- What is their L1s(s)?
- What do the parents expect out of the class? What about the children/students themselves? And the church leaders who organize the Sunday School in the first place?
You don't need to respond with an answer to all these questions, but hopefully this will get you thinking through the right kinds of issues to help you arrive at a good solution for your situation.
Depending on the answers to the above questions, it's actually likely that it's actually not advisable to focus only on explicitly teaching grammar. Instead, it might be advisable to incorporate some more authentic communicative activities in the classroom, too.
Finally, if you do an Internet search for "ESL children" or the like, you can find many websites with teaching materials, lesson plans, etc. that you might find useful. Some that caught my eye were esl-kids.com, eslkidstuff.com, eslkidslab.com, teachchildrenesl.com, and funenglishgames.com. Also, some general-purpose websites for (mostly adult) ESL have special sections for teaching children - e.g. eslcafe.com, eslbase.com, and rong-chang.com. If you have any means of bringing technology into the classroom, some of these websites have games/activities that you could let the the children access and use themselves to practice.
I hope this helps!