Although it's been a number of years since I've been teaching and I don't have Susan’s level of expertise or your number of years experience, I feel your panic as you describe your upcoming new class so I thought I'd share my experience.
When I was teaching ESL in South Korea I spent a couple of years teaching mostly kindergarten. One class in particular was mixed with 3 to 6 year olds. There was even a child who showed up one day pantless (no diapers and no pants, that is) but I turned him right around back to his mom. Pants not optional!
In my first year of teaching I did not know enough to require a textbook from my school. With that mixed age class I learned that the parents’ main goals were simply to give the children exposure to English (and to "foreigners") and to have the kids associate learning English with fun.
Later I worked at schools with more structure and yes, having a course book was such a relief. One series that I particularly liked was the Let's Go series (Oxford) because of its supporting materials-- engaging activities and useful photocopiables in the teacher's manual and, especially, additional audio materials with a book of songs and chants which was a godsend for teaching young ones. The chants were as catchy as TV commercials. A decade later and I still can't erase a few of them https://elt.oup.com/catalogue/items/...ge=en&mode=hub
I think this series may be American English and possibly focused on ESL in Asia, so perhaps not suitable for you. But remember to evaluate the accompanying audio materials when you pick a course book, they were so so useful (and some series have atrocious voices). I cannot sing worth beans or read music, so without the audio I'd have been terrified. Music in the classroom was always a hit with kids, at least in South Korea where people love to sing. I'm sure your students will love your guitar!
In my classes with young students, what worked best for me was to divide the lesson into blocks of time and keep the structure (mostly, flexibly) the same throughout the course. For example, I chopped a 60-minute lesson into roughly 15 minute blocks each of:
1 - songs/chants
2 - new language presentation & activities
3 - out-of-your seat type activities
4 - sit down activities (writing, drawing, making booklets, etc.)
Within the song segment, once we had a repetoire built up, we'd do songs they were familiar with as warm up before introducing the one new song/chant that matched the target language of the day. Singing included actions, gestures, sometimes "solos", half the class singing while the other half acted out, different voices (quiet/loud, happy/sad etc.), and other variations.
In the "writing" segment, yes with 3 to 6 year olds some kids could only scribble and some could spell their names in English, but they could all take home something tangible to show their parents that would cue the words and letters they'd learned that day.
The author Penny Ur has tips for teaching mixed-ability classes. I'm not sure if she discusses the primary age group specifically, however. This is an excerpt from '100 Teaching Tips': https://assets.cambridge.org/9781316...85_excerpt.pdf
I guess for me, the keys to enjoying the lessons with very young kids were variety, movement, interactivity, singing, and flexibility. And being silly.
I hope your class goes well and you have fun. I'd love to hear how it goes!