If I understand correctly, your problem is that any feats on Bard level 1 can't go towards Dazzling Display, because you need Weapon Focus first, and to get that you need 1 BAB. However once you're level two, you could, but you already spent the 1st level feats on other things.
According to the official FAQ, retraining is a great option for you:
Retraining: Can I retrain a feat to replace it with a feat I didn't
qualify for at the level I originally gained that feat?
Yes. As long as the new feat is a valid feat for your current
character, you can retrain the old feat and replace it with the new
feat.
For example, if you are a 3rd-level rogue who took Improved Initiative
at 1st level, you can retrain that feat and replace it with Weapon
Focus. Even though Weapon Focus has a prerequisite of "base attack
bonus +1" (which means you couldn't take it as a 1st-level rogue), it
is a valid feat for your current level (3rd), and is therefore a valid
choice for retraining.
(Note: Likewise, the fighter class ability to retrain fighter bonus feats does not require you to meet all of the new feat's prerequisites at the level you originally gained the feat.)
Here's how that would look for a 2nd level human Bard:
Costs
10 × your level × the number of days required to retrain
= 10 gp/lvl/day x 2 lvl x 5 days/feat x 2 feats
Total cost: 10 days @ 8h/day and 200 gp / 2 feats
Note also that
Some retraining options require you to work with a trainer. If no suitable trainer is available, the GM might allow you to retrain yourself by spending twice the normal time. Even if you train yourself, you must still pay the cost for training (though you don't double the cost as you do the time). Any option that requires a trainer also requires some kind of training facility for that activity.
Aasimar are addressed in Blood of Angels. "Women that carry an aasimar child report easy pregnancies and deliveries..." There's an entire page of info on Childhood, another on Adolescence, etc. They don't come out and say it explicitly but there's no sign of any time disparity; they are described as maturing to age 5-6 like other human kids and having some issues during puberty with their peer group. BoA says, "An aasimar might spend a good portion of her childhood thinking of herself as human." This is odd as the Advanced Race Guide indicates that the adult age of an aasimar is 60, which would seem to indicate there's some kind of slowdown between birth and there... Same situation for oreads etc.
Bastards of Golarion has nothing to say about this for the other races, it's more of a crunch book really.
I think this falls into the general category of "poorly thought out things in the D&D cosmology," which are manifold. Just like the high intelligence of many aberrations, it's an interesting note trotted out every once in a while as a plot point and then conveniently forgotten 99% of the time.
So your playbook:
- Decide if you care - it's a magical world and these are magical crossbreeds, there doesn't have to be one answer and you can be as inconsistent as you want
- Decide if you just want to say "they mature at the same rate as anyone else" and ignore the adult age listed in the books
- Decide if you want to let players decide based on their own concept of their PC's background
Best Answer
Use the Young Template
Young, a simple template, is pretty much exactly what you're wanting. Take an NPC class - I suggest Expert or maybe just a single humanoid hit die - and then apply the template to the resulting creature. Voila, instant kid.