Bettas are so much fun, they can be very interactive fish given the right environment. I have mine in an 8 gal planted tank and he is the coolest thing ever, swims right over to the glass when I walk by.
You will definitely need a heater, Bettas are tropical and are more active when the tank is at the right temp.
Also, it might be a good idea to test your decor by running pantyhose over everything. Bettas have delicate fins that can snag/rip on rough decor. Plastic plants aren't recommended for them for this reason.
At 5 gallons, I would recommend cycling, because that will just make things easier for you in the future, the tank will be lower maintenance that way.
A great site to look into is this forum, which has tons of info on bettas!
There are tons of different colors and fin types for bettas. If you want your daughter to have plenty to choose from, call and ask your local pet store when they get their fish shipments in, you'll have more fish, and likely healthier, to choose from.
Good luck and have fun, make sure to post a picture of your new little fishy!
Sadly our first Beta- Doodle- died of handlotion poisoning at the hand of my 2 yr old. He lived for more than a year. Im now waiting till boychild learns positively not to "wash" the tank for me. I am surprised at how much I miss our little fish friend. He was a great pet. Hopefully your toddler has more success. We also had one giant snail. Our snail was very interesting to watch mouth his algae pellets. I am looking forward to having another, when the time is right.