Buy aquacultred captive bred, from a breeder not from a store.
Keep the tank temp below 74, use a chiller if you have to.
Provide plenty of places for the seahorse to hitch.
Setup a feeding dish.
Skip crabs on the cleaning crew and go with a mixture of snails.
Pick your tankmates carefully.
Design the tank so the flow in the tank is broken up through several outputs. Spraybars under the rocks help you get away with a lot IME.
Oh, teach the girl how to do the water changes, and how to use test kits.
