The Sunny Beach grand is right on the beach road, very near to the center of the resort. Ive never stayed at this hotel.
Sunny Beach is great for all ages and there is plenty to keep the kids occupied. There are 2 main water parks, Action park in Sunny Beach and Aqua Paradise near Nessebur. There are free buses from all over the resort to take you to both parks. For the 5 year old, you may prefer the Pirates of the Caribbean , which is at the Hotel Kotva, at the turning to Nessebur. A local bus will take you there, get off at the football stadium.
Hotel food in the majority of hotels is not the best and many find that they eat out at the excellent restaurants in the resort. Very cheap to eat and drink, but cheaper away from the beach front. Avoid McD, KFC, Burger King etc.