It sounds like your 15-gallon community tank has been doing well the past few years but I don’t think it’s the right set up for seahorses. Your concerns regarding the size of the aquarium are valid. A standard 15-gallon aquarium is really too small for the larger breeds of seahorses such as Mustangs and Sunbursts (Hippocampus erectus) or Spikeys (H. barbouri) or Brazileros (H. reidi). The larger species would have trouble mating successfully in such an aquarium due to the lack of water depth and would be prone to problems with gas bubble disease for the same reason.
There are three species of smaller seahorses that you might consider keeping in a 15-gallon aquarium, but I don’t think any of them would work in your community tank for various reasons. The species that might be appropriate for an aquarium that size are the Zulu-lulus (Hippocampus capensis), the short-snouted seahorse (H. brevirostris), and Pixies or dwarf seahorses (H. Zostera). But it’s not possible to keep the tiny Pixies in an aquarium with clownfish or tangs, and the other two species (H. capensis and H. brevirostris) are temperate (cool water) seahorses and would require an aquarium chiller to keep the water temperature in their comfort zone.
Another thing to consider is that both the clownfish and the tang would certainly out compete the slowpoke seahorses for frozen Mysis in such a small aquarium. In larger tanks that’s not an insurmountable difficulty at all — you simply feed the clownfish and tang their fill of flake foods or whatever they fancy first, and then target feed the seahorses with the frozen Mysis — but you would really have your hands full managing that in an aquarium as small as your community tank…
In any case, jomo, a 15-gallon aquarium with a pair of clownfish and a tang is probably already housing all of the fish it can safely support. Adding a pair of seahorses to the tank may exceed the carrying capacity of the aquarium, and it would be a shame to disrupt your little community tank after it has settled into a happy state of equilibrium for the last three years. If you want to add any new specimens to the 15-gallon community, I would suggest that you consider some of the interesting invertebrates instead — perhaps a decorative shrimp or a small arrow crab (Stenorhynchus seticornis) or a couple of small hermit crabs that would be useful as scavengers.
But when it comes to the seahorses, your best option may be to set up a separate aquarium especially for these amazing aquatic equines. The smallest aquarium you should consider for a single pair of Mustangs or Sunbursts (H. erectus) is a 20 gallon Extra-High All-Glass Aquarium (20"L x 10"W x 24"H), which at least has the height the seahorses need to thrive. Please let me know if you feel you might want to set up a dedicated seahorse tank for your daughter, and I would be happy to explain how best to proceed.
Best wishes with all your fishes!