That's a reasonable but difficult question to answer. Depending upon a child's background, trauma, personality, and a number of other factors, there will be issues that an adoptive family will have to confront. Your family (you and your husband, children) will have to determine what level you are prepared and capable of dealing with.
Read up on varying issues. If you go to
www.adoptuskids.org, it will give you profiles on children, listing their behavioral, physical, mental, or other challenges. Typically, you can "click" onto a behavior listed and another screen pops up telling you more about that particular issue and what to expect. Though you probably won't find many 3-6 year olds on that site, it will still be very helpful re: terminology when you begin talking with a social worker about any child.
The few social workers I've spoken with thus far have been very open and honest regarding a particular child's specific needs.
For me, personally, I am more prepared to deal with the effects of a drug exposed child than one that has fetal alcohol syndrome. HOWEVER, there are others that have adopted children with FAS with great success. This is just my personal preference.
Look very honestly at your limitations. Even those 3-6 year olds may have been through pretty severe trauma and loss which effects behaviors and outlooks. Don't be afraid of that. In reading many of the posts on this forum and speaking with other adoptive families, once a child is in a stable, "forever" home, they many times make remarkable strides. Pay attention to their "labels" but don't allow that to scare you away. Each child and case history is different. Even siblings respond differently to the same crisis.
If it's in your heart to adopt, pursue it. Family members have good intentions, but if we all listened to our dear loved ones, most of us would probably be talked out of the adoption pursuit.
It takes effort and committment to follow through with adoption, even in the best of circumstances. These precious children are worth it though.
Best wishes!