Are your kids morning people or night owls and are you dealing with any time change? My DD(7) has never been a morning person. We tried a pre-park breakfast once when she was little and it did not go well- and we only had a 1 hour time change. We discovered that getting her to the park before rope drop leads to a tired cranky kid. For her, from those options we would choose dinner, but when she was small, not too late of a dinner. If your kids are early risers a pre-park breakfast would be great.
As for dinner, ff your kids are early risers and not night owls, they might enjoy breakfast more. They may be too worn out at night to enjoy the dinner. If they are night owls like DD, I would say dinner, but be careful about booking too late of a time. We never took afternoon breaks, DD would just nap in her stroller as needed, and sometimes when she was little if we booked a table service dinner that didn't start til 6:30 or 7, she would be really tired and restless at dinner. She was fine if we ate CS in the evenings and kept going until park close- but a sit down dinner would let that Disney adreneline shut down and she would feel the tiredness. So...we usually did lunch at CRT! If we did dinner, we tried to make the ADR sometime between 5:00 and 6:00 p.m.