I went with another adult and three kids aged 11, 12, and 13 to SoCal two weeks ago. Ironically, the kids' trip report for the week said that their afternoon visit to TJ was their second highest rated activity (even above snorkeling on Catalina Island & the SD Zoo). For safety reasons, we made certain to leave TJ before nightfall and we parked at the border ($5).
We crossed the TJ border area bridge further into TJ (recommended) and walked around all afternoon. I thought the kids would hate it but they said it was an exciting experience. They ended up bargaining with the street merchants and were engulfed in the different culture. The kids asked questions about haggling, the housing, the tiny streets, the vendors, the carving & glasswork craftsmanship, the young Mexican children running around, the language, you name it -- they were wide eyed. They even began a competition to see who would pay the least amount for a single Hard Rock - Tijuana Shirt ($4.95 was the winner). They brought back bags of unique souvenirs for themselves and their friends along with a fun story about each.
Yes, the language is different, the city is poor, and there is some danger to be respected - but these kids said they came back with a new perspective on what the world was all about. Even crossing back into the US was an adventure and gave them an appreciation for clearing customs.
While I'd agree that going further into Mexico would be better, if you only have a couple hours to spare, TJ can be handled. Be certain to exercise the same cautions (and more) as if you were in any large city. Oh, and if you’re looking for something to drink, be sure to get a Coke, Pepsi, or water (whatever) in a bottle - no fountain drinks.
FYI: The coolest beach, pier, & waves according to the kids were at Oceanside (about 40 minutes north up the coast). The two SD beaches we went to were a little dirty with an oily and glittery substance (according to the locals this pollution comes from TJ's sewer system). Please note I'm talking about the two SD area beaches we went to... I'm not indicating this is the condition of all of the SD beaches.
Finally, the water temps at the time were about 69 degrees – pretty cold if you planned an afternoon in the water.
Let me know if you want more information on what the kids thought. Overall, they had a wonderful, very memorable time in the SoCal area.