I have my shiny gyarados in the day care I hope I can get a shiny magicarp..

I'm not quite sure what your asking but, just because one of the Pokemon in the daycare is shiny doesn't mean that there is a higher chance of the offspring being shiny. I'm not sure what a level 50 Cyndaquil has to do with anything, but if you want a higher chance at breeding a shiny here is what you need to do. Make sure you have wifi and go to GTS and look up a Pokemon you want to breed from another country. Trade with them and put that Pokemon in the daycare with one from your game. It might take a lot of eggs but you'll have a higher chance at getting a shiny Pokemon from that breeding pair.

If you are asking to recieve a shiny magikarp egg from breeding a shiny gyrados and a cyndaquil, then don't count on it.
First of all, if the gyrados is male, then you won't get a magikarp from the egg.

Second, you can't breed a gyrados with a cyndaquil, it just isn't going to happen if they are not in the same egg group.

Sure your chances are a TINY bit higher breeding a shiny Pokemon from a shiny Pokemon in the daycare then finding a shiny Pokemon in the wild, but this doesn't mean it will happen on the first try. You must be ready for whatever is in those eggs and love them as Pokemon or let them hatch and release them.

Hope this helps you.