You could make a card from your current charge and "the bump"
Or write a little poem from your current charge that says I am so blessed to have you as my mommy, thank you for giving me a little brother... or something like that

I nanny for an almost-2-year-old, and MB is expecting again. I would love some ideas for gifts that incorporate my future charge, especially as she is very excited about the new baby (DC was a result of years of fertility treatments so she is still in awe that she was able to conceive again so fast). We are all talking about the baby by his future name already, so I know she really views him as a part of the family (and she is 20 weeks, so not really concerned about miscarriage anymore).