A betta. I am sure you will find a pink, white, yellow, lavender one no problem at a Pets at Home. Just be sure to get a heater. Despite the myth that they can live in tiny bowls, heat is essential... they are a tropical fish!
They need 2 us gallons minimum so one would be awesome for a 4.5g tank. They are also quite hardy... just make sure there are no sharp decorations that could tear his fins. File any sharp corners.
Another thing I strongly, strongly
suggest is that you buy the tank about 5 weeks before Christmas so that you can cycle it (aka grow a colony of nitrifying bacteria in the filter to break down wastes) ahead of time in a good hiding spot lol. Cycling the tank will reduce the amount of water you need to change from 100% 1 or 2 times a week to 25% once a week, which is good for busy moms.
Here's some good articles on cycling your tank: http://redirectingat.com/?id=401X645....php%3Ft%3D343 Freshwater Cycling Methods
Props to you on researching before taking the plunge! I can't begin to tell you how much trouble you are saving yourself. One thing I always respected (and still do) about my mom is that she did her homework on all me and my brother's pets and really set a good example.
(And one tip: Ask us anything. Pet stores often give out a lot of wrong advice which causes a lot of trouble for newbies.)