When talking about medication during pregnancy and nursing, you need to weigh ALL the options- including the dangers of not treating your condition at all. In many cases, the potential side effects of the drug are safer than the risks to the baby if Mom's condition goes untreated. For example, when I'm depressed I often "forget" to eat- this lack of nutrition can be far more harmful than medication.
IMO, cannabis is probably a lot safer than many of the pharmaceutical drugs that are used to treat Bipolar Disorder. However, there are certain "legal risks" of using cannnabis, and you're more vulnerable to the law once you have kids. There IS a risk of being arrested and separated from the baby, and also a risk of children being taken into foster care simply because of parents' drug use. I'm not sure how likely that is to happen, but it IS a risk.
In the "Finding your Tribe" area, there is a tribe of MJ using Mamas and Papas. You could probably get a lot of info and support over there.