Some of my best first projects were vests- they knit up quickly and I didn't have to face the psychological challenge of doing the second sleeve. (I have a project or two just waiting for me to do the second sleeve.
I think it is a great idea to get a yarn and pattern that have been "matched" for you so that you won't be struggling with that process.
A word to the wise... Perhaps pick a size larger than where your child is now- if your gauge is a bit off and it comes out small you still can have your child wear it- and if it comes out big then they just get to wear it longer. :D Or, if it takes you longer to knit it than you expected then your child is less likely to have "grown through" it.
I did a vest for my dd1 and it turned out to fit my dd4.
Keep us posted of your progress!