A good size sponge filter typically has a 1/2 inch tube going through the sponge down to three quarters of the way. An airline is inserted halfway down into the tube. As air rises through the tube, water is carried along with the air and exits from the top of the tube. This airlift movement siphons water through the sponge trapping detritis in the sponge.
A sponge filter is great for clownfish babies that are 30 days old or older. If your seahorse babies have settled and are hitching to macroalgae or some other objects well, I suppose you can use a sponge filter without any problem.