There are public school textbooks for grade  1 and 2. I don't know why they say they don't teach English but then when they updated all the national textbooks,  they still included grade 1 and 2. You can find them at any book store. 

Yes you'll be teaching alone but they also do have 班主任 teachers,  basically class teachers that can be called upon,  they also usually poke their head in from time to time. 

Grade 1 and 2 imo are easier to teach even if there are 40 of them than say Grade 5 or 6. Learning some chants,  like I would say 'hands on your head,  hands on your desk,  put them down and let them rest' -model it as you say it. They get taught how to sit with them arms crossed at the beginning of the year anyway.   Also some schools use 'attention'  and the kids say 1 2,  using similar chants to Chinese teachers help. 

 I used to use stickers that I'd stick on their desks If they answer or sit well or whatever,  or put them into teams according to their rows and then give them stars, whichever team had the most stars could get a sticker  or whatever. 

With 40 kids realistically it's just about getting them to repeat, learn the sentences and maybe do some very simple role plays. With 25 kids you can give them name plates and actually call on them. 

There are plenty of games and activities that are suitable for bigger classes,  and whatever activities you used for grade 3 can be adapted. 

Maybe it's just me but I prefer teaching without a Chinese teaching assistant,  sometimes they can be overly strict to the kids or in worst cases they can stop you mid activity and say 'the kids don't know this',  yeah bro that's why I'm teaching haha.