Congratulations on your new arrivals! It’s best to be patient at first and leave the aquarium light off to give the newcomers a sense of security while they explore their new environment. Seahorses see very well under low light conditions and the ambient room light will more than suffice for them.
So I would suggest just leaving the aquarium light off for the rest of the day and letting the seahorses settle down and and get accustomed to their strange new surroundings at their own speed. Don’t attempt to feed them for the first day. Just give them plenty of room and allow them to settle in and investigate their new home in peace and quiet. Admire them from afar. The next morning you can turn on the aquarium light at the usual time and offer them their first meal.
If you can’t resist, it is safe for the seahorses to turn on the aquarium light at any time after they have been successfully acclimated to the tank, but I like to allow them a little privacy and leave the light off for the first day well they are getting adjusted to their new home.
Best wishes with all your fishes!