@Elizabeth-Rose I agree with the advice already given.
I was really wondering about having to build the garden beds, too — since they had already been gardening year after year. Could you change the text to make it more clear? The mom could say something like, "Instead of working in the big garden this year, I thought you'd like to build a little garden all your own."
You could show the little squirrel struggling to clear out weeds.
I think it's pretty clear that the mom is very ill, since it says in the beginning that she's been sick all winter, and now she's not planning to garden as usual this year.
But if you want to add more to show why the mom wouldn't come at the end, you could have the kid invite the mom to see the progress in the middle of the story & have the mom say, "I need to rest now. You can show me the garden another day. I'm sure you're doing a great job." or something like that. Or have the little squirrel say, "Mom has been very tired lately, so I brought her some flowers to cheer her up." & show the Mom in bed.
Another thought: If the mom has to rest all the time, why wasn't she in bed when they brought the party over? She couldn't have been at the doctor, unless in was an emergency, since the party invitations were sent out & the daughter was expecting her to show up. If she was elsewhere in the house, she would have seen everyone coming through (and be more likely to make it out to the garden). The only thing I can think of is if she was in the bathroom, and even that would mean some coincidental timing that they happened to bring it over — right when she was indisposed. You could consider changing the "Oh, my!" scene to her reacting to them starting to come in.
.