this sounds like a really exciting class; I hope you'll enjoy it!
Do you have an idea of whether they are complete beginners or whether they have some knowledge of English? My guess is that they know some English already from working in an English-speaking environment. This will affect your teaching approach in the firs t few lessons: you will probably cover the same material but will do it faster if they are not absolute beginners.
You could start by presenting or reviewing introductions such as: Hello, How do you do? “my name is “, “I’m a gardener”. This will allow you to see how much English they know and get to know a little about them. You could also review numbers and the alphabet, possibly by playing bingo and asking them to spell simple words. You could also present some basic useful language; the picture dictionary sounds like a useful tool for teaching vocabulary.
You don’t have much available in the way of materials; could you order a course book for beginners in English? This would save you a lot of work in the future; that way you won’t have to invent and create your own lesson plans; the syllabus will be provided for you by the book. You’ll need to do a lot of presentations and revision before you can get to your goal of conversation lessons: dialogue will be fairly limited at that level. Again, a coursebook will help you with that.
You could also look at these posts:Teaching beginnersWhat to teach a student who's going to the USA?Long one-on-one sessions early in the a.m.
Please write in again if you want more ideas.