Fillets of ling and ocean perch have no small bones that would bother children, only large bones that can be easily seen and removed before cooking. Flatfish, such as fillets of flounder and sole, also have no small bones but have sweet flesh that appeals to children. Fillets of coral trout are also suitable.
Putting the fish into a potato-topped pie is a good idea because you can check for bones when skinning and flaking it. Encasing boned fish in pastry is another option; try cutting the pastry into a fish shape and marking scales on it. Suitable fish are bream or snapper fillets; salmon is particularly good as the rich color contrasts well with the pastry.