Well.... your question seems simple enough and is to the point. The answer isn't. To accurately answer it, what is the brand name and model of the aquarium? What came with it? Filter type if any? Heater? Lighting, incandescent, fluorescent, any at all? What kind of water supply do you have, well or water company?

1. First decide what is best for your child.
You really have two major options as far as fish are
concerned: a couple of fancy goldfish or one betta.
These would be the easiest and the showiest of the fish
that a child could take care of for themselves with
basic instructions.
2. Forget about all the fancy appartus for your small tank
3. Fill the tank with tap water and let it set for a couple
of days. Also fill up a gallon jar with tap water that
will be used to replace water in the tank.
4. Put the tank in a warm room but not in front of a window
but be sure the area is well lit so your child can enjoy
the fish.
5. Take your daughter and go to a local pet store and have
her look at the gold fish and at a betta to see what she would like between the two. Tell her that if she is
interested in any of the other fish that she needs to
learn how to keep the two choices first as you would
have to buy a much larger tank for them, if she asks
about the other fish she would see.
6. Be sure to buy the appropriate fish food for either the
goldfish or for the betta.
7. While you do not need to get one you may want to look at
an aerator. Talk to the clerk at the store about one
suitable for a small tank. Needed with the aerator MAY
be air tubing and an air stone.
8. Take the fish home and set the bag with the fish in the
tank of water and let it set for 20 minutes before
opening and letting the fish out.
9. Give the fish a couple of hours to adjust to the new
tank. If they look happy then take a pinch of food
(amount held between your thumb and first finger) and
drop into the tank. Do this only once a day.

10. Every three to four or five days remove 1 gallon of
    the water from the tank and add in slowly the water
    that has been sitting nearby. Dump the old water out
    into your flower beds and add fresh water into the
    gallon jar to set until the next replenishing.

This is a basic set up without all the fancy stuff and all
the hassle of gravel, filters, and plants that make it just
too much of a chore for a child to have to deal with.
Keeping it relatively simple will give your daughter a good
chance of learning and enjoying the experience.
To help you may want to sign onto TetraCare. Check the link.

Here is a link that might be useful: TetraCare

2 gallons. You don't have much choice, if you want to be humane about things you can only have 1 small betta in it though they should really get 2.5 gallons or more. Or you could put a few ghost or red cherry shrimp. Anything else would be too big and inhumane for such small tank.

You will need a heater and filter for a 2 gallon.

Or you could put some aquarium gravel in it, drop in a goldfish and let the kid enjoy it. If the fish should die, blow another .25 and put a new goldfish in it.

If you have city water, you should let the water stand for a day or two before adding the fish. This will allow the chlorine to gas out of the water and into the air.

You do NOT need a filter for a 2-gallon aquarium.

I suggest keeping only 1 betta fish (male or female) in this size tank. You'll need a heater for a betta because they are tropical fish.

I would use gravel (a.k.a. substrate) and your choice of live or silk plants so the betta would feel more secure.

Of course, you'll need to make partial water changes every 4-5 days (20% of the water removed).

IF you decide to get a betta, make sure you get 1) water dechlorinator and 2) aquarium salt to keep your fish healthy.

Bettas are very easy to care for, and a 2.5 gallon tank is ideal for only one betta fish, no more. Mine is thriving for 10 months now in one.

TammyPie

whatever you do please don't put a goldfish in there!

i would only put a betta in there as well...

if it has a light how about trying a small live plant to help out with the no-filter problem.

Lucky bamboo sticking out of the top would be neat

Most heaters are for larger tanks. I was thinking of getting a small tank for a Betta instead of a using a bowl like I usually do because I wanted to heat it somehow. Most of the small ones in pet stores don't come with a heater and most heaters are for 10 gallons and above. If you don't heat it it would be better to get a goldfish. Maybe just "rescue" a feeder goldfish.

Hi everyone.
I've seen some good advice here. but one thing I haven't seen mentioned is that 2 male Beta's will fight to the death. they cannot go together. and a male and female not good either. the male will want to breed, and chase her to her death in such a small tank.

I've kept plenty over the years in bowls.. they are wonderful colorful fish!

JoJo

10 gallon aquariums are only about $30 where i live...i'd invest in this for your daughter...perhaps?

in my opinion it's just ignorant and cruel to keep anything in a tank so small

it's like buying a Chihuahua and keeping it in a dog carrier just because it's a small dog and can survive in there. just because it's surviving does not mean that it's happy and healthy.

sorry if my honest opinion isn't what you want to hear, but honestly is always best regardless.

how you are maintaining

Here is a link that might be useful: cheap parts accessories