You're a youngest child, huh?
Second youngest, but it was usually my mom forgetting to wake me up or tell me if something was happening ╮(￣▽￣)╭
She's the youngest child and she probably would have messed a lot up, not to mention she's like max 5, she needs her sleep. Plus it's not that deep in this situation because they made sure to still include her and save her her own part. Don't worry, I understand where you're coming from and thankfully they made sure to include her and also think about her feelings after she was woken up so it's fine in this situation
If you're not the youngest, youre a middle child. So, yes, middle children are often overlooked. Youngest do get their wants catered to, but it's usually the parents. From experience, the siblings see us as exhasperating hahahah. They probably were working on a tight schedule and could not wait for her to wake up.
Why did they start the cake without her, like making the actual cake sure, but the decoration bruh she was probably the one most excited to to that part and the older siblings just seem to downplay any feelings of disappointment she has for not being included in things.