I'm not great with fish, but I gave dd plaice mixed with mashed potato and cheese when she was 6 months. How about fish pie, as well?
Plaice is a really good first fish because it is so mild and easily checked for bones. Just buy a fillet, cover with milk (not too much, though) and clingfilm and bung in the microwave until flakes off easily. Then mix with whatever.
For fish pie, make a cheese or white sauce, mix in fish chunks and any veg i.e. broccoli or peas and cover with mashed potato and bung in the oven. Annabel Karmel has quite good fishy recipes in her cookery book. Also, I used unsalted tinned salmon mixed, again, with mashed potato (dd has mash coming out of her ears) or cous cous, and that's good because the lumpiness is easily controlled.
I never know how long things last in the freezer, I'm afraid. Need a good clear out really, because is jam-packed and I'm sure there's stuff in there that's been kept for AGES. My problem is I buy lots of lovely fish and meat and then don't cook them by the expiry date, and so shove them into the freezer for another time, which never comes round as can't be bothered cooking if can't freeze up a batch for dd.