Buying seahorses, or any pet for that matter, for a 4 yr old is unrealistic. If you want seahorses, and you want them to be in the 4 yr olds room, then by all means, get them, but don't try to come across as if the 4 yr old will be taking care of them or doing any maintenance, because he won't. And if you actually expect that from a 4 yr old, then my suggestion is to get a 10 gal tank full of guppies or something and skip the seahorses. That way, when they die, you won't be out so much money.
Now, if YOU want seahorses, I would say a 29 gallon is the minimum size for a pair or trio. It's easier to keep the water quality in check, and there is more room for natural landscaping. Seahorses don't do well in a nano tank - it's just a plain fact that they poop too much. A way to combat high nitrates and phosphates is to add caulerpa, which helps with water quality, is attractive, and a natural hitch for the horses.
Tankmates should be kept to a minimum. Some small gobies are OK. Most softy corals are OK. Otherwise, it should be pretty much horses and horses only.
Flow will be relative to the size of your tank. A happy, healthy horse loves to play in the flow, but none will do well being blown around the tank like a leaf. There should be some very low flow areas, especially if you are going to use a feeding station, but a flow which makes softies and caulerpa wave around is OK.
Please, please, please buy only captive bred (CB) horses, especially as a first time owner. Those generally considered the hardiest and easiest to get are erectus. If you don't want to mess with babies, which are not like raising guppies by the way, then get same sex horses. Buy all you want from the same source at the same time. This cuts down on the possibility of transferring disease.
In case you didn't know of them, there are also 2 very good, informative sites -
www.seahorse.org and
www.syngnathid.org. There are 2 very well respected erectus breeders on seahorse.org - Bruce and DanU. Either of them should be able to get you some beautiful, healthy horses for a reasonable price, and you won't have to take the LFS word for it that their "black" seahorses are CB and eating frozen.