I have had all of these pets...and almost all "domestic" and tons of exotics before also though..!! Plus I was going to school to be a zoologist... anyways,
Turtle, Great easy pet. Food is cheap. Need an aquarium that is large enough though...although how large depends on the breed. I have a red eared slider boy for example. He had a 30 gallon tank and that works great...Downsides, you have to have one heck of a filter! Turtles stink! They excreet acid basically for their movements. So you have to constantly change expensive filters or your house starts to smell rank. Also, the vast majority of all breeds carry salmonela. So your children have to learn to handle them with care or they will get sick! The younger and smaller the turtle the more concentrated the salmonella! Never get a baby turtle! Most states don't allow turtles under so many inches to be sold but if yours does still pass on babys!! They have a high mortality rate and can be very deadly salmonella wise! They also require a high amount of husbandry which most don't know but they do! Aside from those downsides if you learn good husbandry and hygeine they make wonderful pets! I adore mine..Plus most breeds live at least 20 years some up to 100!
Also, if you decide on a different reptile most eat live things. fish, crickets, meal worms, mice, rats etc. If you get a reptile that requires crickets. crickets are far worse than the reptile to care for!! They smell, and don't live a long time. So I was at the pet store half the time every third day for my frogs, toads, lizards, things of this nature. Not to mention getting frozen rats, and mice is kinda strange for a little boy! LOL! Although, my favorite pet ever was always my amphibians! They don't live that long unfortunately. only downside to um.
Guinea pigs, Great pets. Not much different than a hamster per say..require a cage...open one perfered. Food is inexpensive, bedding as long as you get a paper one, not wood, reduce the odds of parasites getting your pet and killing it. They are kinda stand offish till they get comfortable with some that is never mind you! But I think this is a wonderful choice for a first responsibility!
Fish, depends on the fish. But always a good choice. Look up the kind you want online and see what they require food wise. All you usually need is a good aquarium, filter, food, and chlorine remover plus the pretty stuff for the aquarium. Change the cage as often as recommended by the filters standards. A very, very, easy pet. But usually a low life span.
Dogs, Well i assume cause you said another you had or do already have one. I would be leary against a dog for a 7 year old cause I know with my knowledge...that usually means mom will end up taking care of it most of the time!! LOL!
And lastly ferrets, You said you are already well versed in um. Just be warned that some smell far worse than others descented or not!! I have been around alot. They go from mildly stinky to beyond wonder how they got that stinky!! So if you can get one the same sex, and kinship to the ones you like I would only go for that!! LOL! Otherwise you might be doing alot more cleaning than for a cat! LOL!! Or a dog for that matter!
If you have question about any other animal. I would be happy to give my opinion. I won't sugar coat it. I don't think it is good for people to get animals that when they don't understand the commitment they are getting behind. Shelters are so badly over run as it is. I am constantly running a zoo. So like i said previously I have had more animals than i would care to list. Although, i prefer reptiles and amphibians to be honest anymore. But I would be happy to help in any way i can. I Have oodles of books on all sorts of animal also.