K.E.
This email applies primarily to your older daughter.
I never had any luck with the Silk Elements product line. Everything we ever bought from Sally's turned out to be silicone based or infused with so much alcohol that it was counterproductive. My daughter is 5 and also has hair to her butt when wet -- we found a product line out of a place in California - www.curls.biz Their stuff is awesome. It is all water soluble, and does not smother or dry the hair - I very highly recommend it. You say you wash your daughter's hair once a week - do you condition it in between? Curls get very dry and thirsty. Every day, (or every other day if you are very pressed for time), she should get in the shower, drench her hair and slather on conditioner. Comb, rinse out conditioner, comb again and apply thick creamy leave in conditioner. On days that she doesn't condition, use an OIL based detangler without alcohol, to get the comb through her hair as well as add much needed moisture. Then apply the creamy leave in conditioner. You will see such a huge difference in her hair, you probably won't want to relax it!
Also - what are you doing with her hair at night? Best advice I can give is to braid it into cornrows and throw on a satin night cap. This helps to keep the hair from getting tangled as well as keep it from drying out as much.
I know it sounds like a lot of work, and it is, but it is sooooo worth it.
For the 9 month old - I treated my girls' hair at that age the same as my own curly caucasian hair until they were about 2 yrs old and it started to get long, more coarse, and nappy.
Best of luck,
K.