Second hand smoke of any sort is unhealthy. Marijuana smoke brings different chemicals to the air than tobacco smoke does and hasn't been researched as closely on young children and how it may affect their growth and development.
I think that, when it comes to kids and smoke of any kind, you can't be too careful. Even though I'm a firm believer that marijuana isn't as harmful as "they" would like you to think, I still think it should be kept away from your children.
Marijuana is first of all in my opinion not a drug, but when it comes to children they should be protected at all times. For them to second hand smoke even the lightest cigarettes just seems like bad parenting.
Any smoking near kids, whether tobacco or grass and tobacco, is extremely dangerous. The children of parents who smoke are at risk of low birth weight, SIDS, asthma and cancer.