it's a rio 180, so 180 litres, (the juwel tanks are handy like that, the tank no is always the litres it holds) knocking on 50gals off the top of my head. nice size tank.
I can understand that you wanna reward your daughter, but I'll say it once more, I'd urge you not to reward her at the expense of the fish. Cycling with fish is dangerous and nowadays is unnecessary. I'm sure you can find another way to reward her, let her pick out all the ornaments and decor for the tank or something.
really if her first experience of fishkeeping is cycling with fish and the liklihood of fish developing disease or dying, then it's not a good way to introduce her to keeping pets.
perhaps WD can give some suggestions for persuading her that it's the right thing to do to not put fish in straight away, i've not had to go through this with kids so can't really help that much.
however that being said, it is ultimatley your decision and I would hate for you to be put off this forum because of it, so if your gonna cycle with fish then just make sure you are armed with the fact and you do it right.
Your target is to keep ammonia and nitrite under 0.25ppm at all times, so a good quality liquid test kit is vital. To keep it down you just do water changes, sounds simple enough yeah, but it'll be every single day, sometimes twice a day. change as much water as you need to get the levels down, change up to 75% of the water at any one time, an hr or so after a water change test the water again, if levels aren't below 0.25ppm then do another big change.... and so on until levels are down low.
hardy fish for cycling are platy's or danios. Danio's would be my preference because if you get platy's you'll probably end up with babies, if the fish has babies I imagine teaching your daughter survival of the fittest is gonna be a hard lesson and you'd want to try and keep all the babies alive, we're talking upwards of 20 baby fish every single month and you can imagine the tank would very quickly get overcrowded at that rate, and then the babies start breeding amongst themselves. Look for leopard danio's, they're leopard patterned so quite cute, can see your daughter liking them (even though they aren't pink!!)
ha ha i can see the gravel breaking you too!! but tbh, it's not that big a job to change it out if you need to at a later date.