Did you use Miracle grow soil or fertilize them? Sounds like it could be an over fertilization problem.

Could be fertilizer, but remember it's just plain hard to get avo's to start. Water has to be perfect. It's easy to let them get a little dry on these hot days and that would do it. It's also easy to get them too moist and they get root rot. A small root rot problem could also look like that. Or salt burn could do it, also common in CA. I would recomend you keep going with the water the way you are. Just be sure the original root ball is getting water, not just the surrounding soil. Remember, at first, there are only roots in the planted root ball, not out in the surrounding soil yet. And again, not soggy either. it's easy for the surrounding soil you back filled with to make a "pot" and hold too much water in the original root ball.
Next, I would put a good mulch over the whole area. It will keep the soil cooler and as it decomposes it will greatly benefit the soil, including buffering the salt content. You can mulch right away if you want to.
Make it through the summer and it should start looking better.

I am in socal myself, mulch makes a huge difference, but avocados are a pain , I lost 2 trees to frost damage, also make sure to paint the trunk white that helps also. Good luck avocados are a lesson in patience and frustration.