help me get started

You mean help her get started, right? Science fair is for kids, after all.

As a has-been science fair dad the best advice I can give: pick a different project. If you want to build a voice activated drawing robot with her at home, go for it. Sounds like a wonderful time and a great project for a girl interested in robotics.

It's a bad science fair project for a primary student. Science fair projects, first and foremost, need to be the entrants own work. They should be able explain the 'what' and 'how' of all the steps and actually be able to do them. "[Dad/Mom]..." can be an explanation sometimes, but not this time. Second, unless it is an 'engineering' fair, it need to contain a testable hypothesis that is, you know, tested. If your project does not primarily involve measuring something, it's almost certainly not right for a science fair. Third, rein it in a bit. You have chosen a huge project. It's the kind of thing that could genuinely take months of your time even as an experienced roboticist. At least for a young kid, pick something where the write-up is most of the work. You should be able to do 90% of the experimental work in an afternoon. It can take longer to finish, but in a 'checking in' kind of way; waiting for mold to grow or an egg shell to dissolve.

Not trying to be a dick, but I really believe sticking with this project is setting you both up for failure.

Dude, if you need your 9 year old daughter's help with this, I think you should try something more basic.

Bruh my ex defended her bachelors with a project exactly like this. It involves a CNC machine (stepper motors, drivers, control board, frame, CNC senders), servoes to pick up / put down the pen so it doesn't constantly draw etc.

It isn't a science fair project and will cost a shitload to make. Voice activation is also another beast entirely.

I built two homemade CNCs for milling, if you don't know what you are doing, then it will take half a year to understand everything, wire it, setup a frame etc. Impossible to do for a 9 year old (and I don't think science faires are for the parents to make shit for their kids)

Is the science fair looking for DIY projects or experiments? If experiments, as I suspect most science fairs are, what is the experiment? What is the hypothesis?

Next, let me tell you a little story about my buddy taking his Significant Other on a camping trip. He was experienced and all about it, SO grew up in a city with no yard and thinks the outdoors is unnecessary. Cue a backcountry trip with rain, raccoons getting into the food, colder temps than expected, and SO will now never give Buddy's favorite hobby another shot. It is ruined for SO forever. If you are trying to get a young person interested in STEM stuff, don't be like Buddy. Introduce them via a light-duty, fun, heavily vetted project you know you can be successful at. Doubly so when there is a deadline for success imposed by the science fair.

There are companies online that sell Line Follower Robot kits. If building a LF robot meets the intent of the science fair, consider one of those kits instead of attempting to design it from scratch.

1. kiddo is introduced to age-appropriate skills and concepts

2. everybody knows an adult helped her, she's getting no boost from a complex self-invented thing that she obviously didn't do herself

Rule 5. Locking.

You might consider doing something with the Lego robotics stuff, something like Nxt-sketcher.

3d printer or desktop CNC with a pen attached and a software layer to translate svg path data to gcode?

A Spirograph attached to a microprocessor controlled motor?

Buy cheap ender 3 3d printer, replace the hot end with a small spring loaded bracket that holds a pencil under tension, write basic x y gcode by hand or make a single line path in a slicer that supports it.

Depends on what you mean by a "robot" but you might look at the ZenXY machine from V1Engineering. It's an open source project meant to be used for drawing in sand but could be swapped and flipped over to use as a drawing machine. You could also Google "pen plotter projects" or something similar as I've seen a few out there that do exactly what you're asking.

As others have said, a 3D printer can do this too if you replace the extruder with a pen and configure it correctly. You're just looking for any sort of CNC controlled device for the hardware. For the software you'll need something that can generate gcode to draw what you want. Sandify is a tool that was created to work with the above mentioned ZenXY in order to create drawings and export them as gcode, but you'd need to do these ahead of time and have them stored on the device.

You mean like those drawing robots they used to have at Chuck E. Cheese that took a snapshot of you and then sketched you?

Tie a texta to a BeeBot, as a prototype.

Like this:
https://www.youtube.com/watch?v=5ArgCB0AmQc