First off you have to read between the lines relative to H&V: people respond to the character meal and the Fantasmic Pkg dinner as though the meals and experience are the same, they are not.
I think the H&V is a must do. You have toddlers, this character meal is geared for the younger kids, you won't' get this opportunity back. Now is the time to go do this one. CRT and CP will still work when they are older. The food will be basic, but filling. The CM and character interaction will be memorable. I gather you selected this for the character aspect, don't second guess yourself now.
Boma breakfast. I think this could be a really good meal. Again there is a bias away from AKL if you aren't already there or on your way there, but it is an amazing resort and the ability to peep some wildlife might be priceless too. Also as you are not staying on site and doing the DP it is one of the least expensive buffets on site. As you've said once you get in the rental car it really won't matter where you go on property. We loved our dinner there and from what I read there are some std and unique items for breakfast.
'Ohana I think this is one that you could indeed wait on given the age of your kids. You may want to read up on it a bit more here on DIS. I seem to recall a lot of "I felt rushed" comments about 'Ohana. You'll need to determine if you read the same things and if that would be an issue for you.
Finally, my advice? Don't cancel a thing right now. Things happen once you get there, you may or may not get CP or CRT, etc. Cancel while you are there and need to make a change of plans. We happened to cancel our H&V breakfast ADR, because at that point in the trip relaxing by the pool for a morning sounded great and there was no way we could have eaten a buffet that AM. We had the H&V Fantasmic pkg booked for the next night and enjoyed that dinner and seating benefit.