Different food, different atmosphere, different prices, different experiences.
CG as a Signature is a good bit more expensive. Food quality and service quality are/should be higher end. Menu versus family-style, all you care to eat.
Personally, I like Ohana very much. And if you are on the basic DDP, I don't think a Signature is a good financial value. If you are on DxDP or are paying OOP, then Cali Grill may well be the better choice.
Another vote to keep California Grill, watching wishes from the roof of the Contemporary is a wonderful experience. As others stated the food, the view and atmosphere are great and your kids will remember it for a long time.
Keep in mind both of these restaurants normally take 90 minutes to eat at. Plus you may have to wait for tables. So I'd consider whether either one is worth finishing dinner at 11pm at night.
We ate at Ohana for the first and last time this year. I loved the desert and the cocktails were good but the rest was not good at all. View of fireworks from the beach at the poly though is amazing.
Cali Grill is on the top of our list every year. Food and service always outstanding and we ate there twice this last trip and both meals were out standing.
The Wave is another great option if you want a good TS meal and its only one credit. Its at the Contemporary as well. Sorry to throw another one into the mix but if you don't want to waste 2 credits then that might be an alternative for you.
I'd go for Ohana. Your kids will enjoy the entertainment and atmosphere more... and you can also hit up another TS restaurant with the credit you saved
Ohana is good, California Grill is great. The only downside is that CG can be a loooooooong experience, and with a 9:30 ADR, your little ones might be too tired to enjoy it fully.