I don't know why the girl didn't consider becoming a maid. But ultimately, the manga needs to be extended somehow. She didn't want to be 5 years old; she wanted to be 10, but the potion didn't work. Also, the other children are afraid of the duke's dark energy, and he adopted her because she resembled his deceased wife, not just because she was brave.
I think their household is like closed off and no maid recruitments had happened and only the position as his adopted child was open...and as for the male lead
SPOILER ALERT!!!
Since he becomes the same age as her in later part like that will be cleared up and there's no huge age gap between them or something

I have sooo many questions, why did she think it was a better idea to infiltrate the manor by transforming herself into a child and posing as his five year old daughter instead of just infiltrating the manor as a servant by changing her appearance? Let's be fr, a servant would have a lot more access to information than a 5 year old would.
And secondly why is her male lead a full ass grown man while she's currently 5? The author should have waited to introduce him after MC returned to her usual adult self
Thirdly, it was so dumbb how she ended up getting chosen for adoption, like she was really the ONLY bold child he met after going through dozens of orphanages in the entire country? Like be fr, lots of kids are very precocious, if bold is what he was looking for, he would have found that long before encountering her. This entire story is just weird, everything including the plot line, world building and storytelling is a mess. The only thing half way decent is the art