As far as I understood you are supposed to teach using CLIL ( Content and Language Integrated Learning) approach- you can read about it in Internet. Anyway, I think that you should try and do your best. You have to know the curriculum of the other lessons- that �s obvious. You don �t have to speak English all the time ( if needed you can use their native language)They are small children so the material won �t be difficult. You will teach them numbers, counting- which can be connected with learning English:  count how many cows are there in the garden: If one of them goes away how many cows will stay etc, you can connect it with reading stories and other interesting activities. I think that �s a very intersting offer. These are not diplomas which are the most important- remember it :)If I were you I would take it. Good luck.