IF you were going to use a BC to house seahorses in, then a 29 gallon 'cube is the....least problematic.... biocube to use. IIRC the dimensions of a 29g biocube are somewhere in the range of 20' x 20' x 19" tall..... and the height is what will become the problem for you kiddies. H. erectus and H. reidi are both fairly large species, and will use as much water height as you can give them. I personally feel that a 21 inch tall tank should be the minimum, with a 24 inch tall tank being the ideal for this sized horse. They will utilize the height for things like their morning courtship dance and especially the mating dances.
That being said, I don't think there will be an immediate problem with the height of this tank...but you may want to strongly consider another tank in the future for them. Just think of it as an excuse to get another tank set up...I know I hardly need much excuse to think about that kind of thing!
The biggest concern IMO would be the temperatures in a biocube. I don't have direct experience here, but I've read that biocubes - or any all-in-one system - tend to run a little warm. I've also heard that the HQI lighting biocubes are just plain hot! Either way, you should make very sure that you have some way of keeping the tank temp down. I keep mine at 74 with my open top and mini chiller, and truthfully this is the upper end of the seahorse's spectrum - 70 to 72 is really ideal. Although southern H. erectus are classified as "tropical" animals, they seem to be more active in the slightly cooler water. Most importantly, temperatures in the lower 70's help slow the growth of potentially lethal bacteria that seahorses tend to be so susceptible to.... conditions like tail rot, snout rot, and other flesh-erosion problems are usually bacterial in nature. The lower tank temps tend to keep these conditions at bay, whereas temps approaching 80 seem to ALWAYS cause problems.
If you can find a way to chill your biocube, I'd say you'll be fine for as much as a year. If you decide to keep them in the BC, make sure you get a chiller that will handle a future tank upgrade. And as others have said....... please post a FTS!!! We love them here! ;-)