Here is a PADI Link
Diver Dan's Online Scuba Store - You adventurous but to young to get scuba certified? The PADI Seal Team Adventure program is for you!
It looks like after the pool certification, the OW is available to 10 year olds.
So probably pool for both. Then when youngest 10 OW for both if pool works out.
My daughter 8 freedives to 15' on one breath and will go Abalone diving with me and possibly spearfishing this year or next. We also fish and go crabbing together.
My dad took me spearfishing when I was a kid. I was swimming in the Pacific Ocean when I was 4-5 and have old film of me backstroking Waikiki.
As for SCUBA though, it depends on the child. Dive instructors would be the best. Try SCUBABoard for professional information on children and SCUBA or calling the LDS directly.