I live in San Diego. You're staying on the border of one of the hottest dining areas in the city and are within walking distance to so many great local choices. With all due respect since I'm new to this board (but definitely not new to the San Diego dining scene), you're getting some pretty bad suggestions. No need to venture to Coronado, Point Loma, Old Town, or Downtown proper. Don't even consider wasting time at a national chain restaurant like Ruth's Chris, Cheesecake Factory, Claim Jumper, or even a local chain like Karl Strauss. It would be really sad if you missed a great meal for something like that. Please consider expanding your budget for at least one dinner, because you'll be treated to some of the best food our city has to offer if you do.
Eater San Diego is a great resource and keeps updated lists of excellent local restaurants:
https://sandiego.eater.com/maps/38-best-restaurants-in-san-diego and
https://sandiego.eater.com/maps/best-new-san-diego-restaurants-heatmap. You'll see that Little Italy is very well represented. A 5 year-old would not be out of place in most places.
These are my suggestions in your budget within reasonable walking distance from your hotel (I can make many more suggestions if you are willing to spend up to $25/entree and the food would be on a whole different level):
- The Crack Shack: casual outdoor restaurant with a menu centered around fried chicken sandwiches
- Carnitas' Snack Shack: pork-centric menu right on the waterfront with outdoor seating
- Little Italy Food Hall: many different interesting choices for a casual meal in a food court setting
- Underbelly or RakiRaki: you said you preferred American, Italian, and Mexican but if you want to try something out of your comfort zone and in your price range, both places serve a nice bowl of Japanese ramen (the real thing, which is very different from the dehydrated supermarket stuff with the seasoning packet)
I saw a couple of mentions for Filippi's. A favorite of many and the price is right, but definitely not my preference. Big portions of Italian-American style dishes like spaghetti, lasagna, ravioli, chicken parm, pizzas, salads, etc. Reminds me of the Buca di Beppo chain. Nothing wrong with it if that's what you want, but there are so many more interesting restaurants in the area, including places that do better Italian cuisine (it is Little Italy, after all).