Hi again, blithespirit!
I think it's really awesome that you're so invested in giving your girls their Chinese language heritage. Your husband is really lucky to have not only your buy-in, but your enthusiastic advocacy on this.
I'm also trying to raise my baby to be bilingual, but I'm having a hard time because I'm kind of alone on this. I'm the only person who speaks Chinese to my son on a consistent basis, since my family doesn't live nearby, and my husband and in-laws (who are local) don't speak Chinese. My son (he's 15 months old) definitely understands the Chinese I speak, and he does "speak" a little (all of his speaking at this point, both Chinese and English, is limited to maybe a dozen "approximate" words). I don't use any particular method to teach him, nor is it intentionally structured, but unlike your in-laws, I speak to my baby like, well, he's a baby! So it naturally involves a lot of small sentences and a lot of repetition. I don't know if this is any hope, but like I said, it's just me who speaks Chinese to the baby. And I stay at home with him, but I'm really only speaking Chinese to him when it's just the two of us. When my husband's home, or when we're, say out and around non-Chinese-speaking people, I speak English to him. So, it's really not a very large proportion of his life that he's being spoken to in Chinese. But in spite of all that, he's definitely picking some up. So maybe you can tell your husband that it doesn't necessarily mean he has to converse with your daughters *constantly* in Chinese or design some special program for them, but that they can learn without a massive educational initiative on his part, just a conscious effort.
Perhaps when you're with your in-laws, *you* could ask them what they're saying in front of your daughters, and then repeat it for your girls? And if you're pronouncing it wrong, I'll bet your in-laws will be happy to correct you! Maybe it'd make a good reminder to your in-laws that their granddaughters are there listening, and they'd love to participate in the conversation? Also, maybe you could look for CDs of Chinese children's songs that you could learn along with your daughters. Then, maybe if your girls share what they've learned with their grandparents, it'll spark some Chinese sharing that's more on their level? I've got this book that comes with a CD (http://www.amazon.com/Chinese-English-Nursery-Rhymes-Languages/dp/0804840946/ref=sr_1_1?s=books&ie=UTF8&qid=1295074847&sr=1-1), and when my mother listened to the CD, she was so excited to hear a couple of songs she remembered from her childhood and it made her want to teach my son more nursery songs in Chinese. Maybe another idea along the same lines is a book like this (http://www.amazon.com/House-Flap-Book-Lift---Flap/dp/9812680365/ref=sr_1_1?s=books&ie=UTF8&qid=1295074999&sr=1-1). It's a picture book with flaps, and each flap has the name of the object in both Chinese and English. My son loves this book. I can ask in Chinese, "where's the window?" or "where's the blanket?" and he'll find it! A nice thing about this book is that it's got a lot of everyday objects like things around the house that your girls can quickly learn to identify in their daily lives. So, it could be a way for your girls to initiate Chinese conversations with their grandparents, rather than just have to be passive observers of the language. Hopefully, if your daughters are the ones pointing things out and identifying them in Chinese to their grandparents, it might help bring the conversation to their level. Because maybe your in-laws just don't really know what you mean by talking to them like they're kids; but if your girls initiating by pointing at the mailbox and saying mailbox in Chinese, etc., they can get a better understanding of where your girls are at (maybe, "Oh, okay, I see that what they're into right now is knowing the names for things that they see... hmm, maybe I can share with them the names of things here in kitchen!" or "Oh, you know the word for mailbox! Hey, what goes in the mailbox? Do you want to know the word for "letter"? For "mail carrier?").
What I'm likely going to look into soon is some Chinese immersion playgroups/classes in my area, just to reinforce and add to what I'm teaching my son (face it, I need backup!). Is there anything like that available where you live?
Again, blithespirit, I think it's so wonderful that you want to instill your daughters with their entire rich linguistic history. These are very fortunate girls.