We took our 7-yr old. It was an 8pm showing on a Sat night back in March. I purchased the tickets the previous JULY - I asked for the first Sat night with four orchestra seats together, and that was the best I could do.
He dozed off after the intermission, and I woke him at the end. If I do it again, it will be a matinee. He did like what he saw; I never dreamed he'd crash halfway through it. In all honesty I wouldn't take a 3 yr old.
As for getting there from Anaheim - If you're doing a matinee, try this - take a taxi to the Green Line train station just off the 605 freeway in Norwalk. Take the green line train to the blue line to the red line, and get off at the Hollywood & Vine station; the Pantages Theater is just across the street. Do it all in reverse to get back. (don't try this at night; I don't think the trains run that late + not safe) Taxi fare to train station will probably be $20 each way (just a guess); train tix will be $4-$5 per person. Give yourself at least 1.5 hours of travel time on the trains; i.e., leave your hotel at least 2 - 3 hrs before showtime.
Another option is to take Amtrak from Anaheim to Union Station in LA, and pick up the red line there. You'd have to research fares & hours for Amtrack.
If you take a taxi from Anaheim to Hollywood be prepared to pay at least $60 each way.
We rode the trains this way (we left our car at the train station for the day) when we saw Tarzan at the El Capitan theater 2 years ago; however the El Capitan was about 1 mile from Hollywood & Vine!! We got our walking in that day, for sure. If you do this, be aware that the Blue Line portion goes through South Central LA.
Another suggestion is to break up your vacation & stay 1 or 2 nights at the Universal Hilton or the Universal Sheraton; you're right there at Universal Studios, and there's the red line station there too!! We stayed at the Universal Hilton the weekend we saw TLK and took the red line to Hollywood; it was just 2 stations & were on the train for 5 mins.
Hope this helps!!