Hi Niniedney,
There's really a lot you can do to help your child's early development. Not all of it has to be about academics. Sounds like you're already doing quite a bit but let's clarify a little first.
Tweedlewink - is this the classes or the DVDs?
YBCR - I'm assuming this is the program by Robert Titzer?
You might want to consider adding physical activities and "bits of intelligence" cards (Doman program). You can buy them from Doman or make your own. Just pick a topic, e.g. Solar system. One a card, paste a picture of the subject, e.g. The Sun. On the back, write down 10 facts about the sun. Make a series of cards - for this example, it could be Earth, Venus, Mars, Saturn, etc. Flash the picture part of the cards to your child and read the name of the card. Flash the cards again but this time read out the first bit of information you have written on the back of the card. Do this three times a day with 4 bits of intelligence cards. The next day, flash the cards and read the second bit of information. It should take you 10 days to get through four cards. Then start on another four cards. You can stay with the same subject or change topics. Follow your child's interest. THis will be great for your 2.5yo. The recommendation is to start at 8 months, but you can see if your 6 mo is interested.
Physical activity - have you heard of Doman's book "How to Teach Your Baby to Swim"? You can introduce swimming to your 6mo. Babies have a natural reflex for swimming because they have been suspended in amniotic fluid for nine months so now is the best time to get her started. Swimming offers other benefits to your baby’s development - because the brain grows through use, swimming provides additional sensory stimulation and opportunity for a baby to grow his brain. This translates not only to physical development but mental development.
Another program is called Shichida/Heguru - they are Japanese based. As far as I understand Shichida came out first but the teachings and methods are similar. Shichida's ideas are based on Doman's philosophy so they are all linked in some way. Some of the Tweedlewink stuff is also similar. The idea is to promote brain development and right brain potential (photographic memory, speed reading).
More about Heguru:
http://figur8.net/baby/2010/01/25/henguru-right-brain-creativity/I take my son to Heguru classes but I find that a lot of the activities can be replicated at home if you're willing to put in the time to make the resources.
As I mentioned earlier, not everything has to be about academics. Jill Stamm - author of Bright from the Start talks about the critical first three years on the development of the brain. The three key requirements of a child are attention, bonding and communication. You can read a lot of the content from this book here:
http://books.google.com.my/books?id=jDLzs0LqlawC&printsec=frontcover&dq=bright+from+the+start&source=bl&ots=xg_msFk30G&sig=5auaN09eESHTTl3F6t8umcl9m30&hl=en&ei=1j3LS9DwH4fCrAfM6Yy5BQ&sa=X&oi=book_result&ct=result&resnum=5&ved=0CBwQ6AEwBA#v=onepage&q&f=falseBronson and Merryman - authors of Nurture Shock - also talk about things you can do to jump start your baby/toddler's language skills:
http://www.babble.com/jump-start-language-skills/index.aspx. According to the research, the way you respond to your baby and the timing of your responses can make the difference between whether you baby learns to talk early or late.
If you follow the Vygotskian approach - also referred to as "Tools of the Mind" - imaginative play (particularly for your 2.5yo) is very important for her development. By imaginative play, they refer to games such as role playing. It helps children develop concentration, focus, and self-control.
Anyway, that's just a start... Hope it helps.