We just started our 8 y.o. DS on Zoloft after several years of trying other (non-medication) options, including play therapy, counseling, cognitive behavioral treatment, etc. His anxiety is always present, but it waxes and wanes a lot, so it has been easy for us to convince ourselves during the good periods that he is maturing and better able to use the strategies, etc. but unfortunately the issues have eventually come back worse than ever each time. His anxiety is not as much separation related, he has fairly generalized anxiety, but the anxiety related to social situations has had a big impact for him (and he also has a lot of difficulty reading others' intent accurately and understanding non-literal language, which also contributes to the social-related anxiety). He also goes through periods where he becomes very explosive when his anxiety is high. We had already decided we needed to go ahead with the medication when his explosiveness increased again, and honestly I wish we had agreed to try the medication sooner, but we really weren't comfortable making that decision when he was going through a relatively good period.
Our DS (8 y.o. and about 62 lbs) was started on 25 mg once per day, and it has just been increased to 50 mg once per day. He has been on it just under a week, so it is hard to say whether it is really helping yet or not, as it can take a while before you can tell whether it is effective or not. Just within the past day or two we have noticed some improvement, but of course that may or may not be related to the medication. He was also put on a very tiny dose of seroquel (12.5 mg 2x/day); this medication is in the atypical antipsychotic class (but is prescribed in doses of over 400 mg for that purpose); at lower doses it apparently has some anxiolytic properties. We will hopefully remove this medication in the future, once the zoloft is working well. As far as side effects, he is complaining of some stomach aches and occasionally headaches, but I know I had debilitating headaches and nausea when I first started on an SSRI and it went away within 1-2 weeks, and his side effects do not seem severe so I am willing to stick it out and hopefully once his body adjusts he won't have these side effects anymore.
On the other hand, we are not currently considering medication at this point for my younger son (6 y.o.) who also has a lot of anxiety (including separation-related). However, he has made more steady progress over the past couple of years with only minor setbacks, unlike our older DS who has the huge set-backs. I know there is a possibility we may have to consider medication for our younger DS in the future as well, but for now he is managing with some very supportive school staff and lots of preparation for things that are anxiety-inducing.
Since we are so early in the medication process, I can't really speak to pros/cons etc. with any certainty, but I did want to reply since I know how difficult a decision this is. Are you comfortable talking to your Dr. about your concerns and the pros/cons? There are a couple of different SSRI's that can be prescribed to kids, so you might also want to discuss the pros/cons of each of the meds. The psychiatrist who prescribed the meds for our DS felt that zoloft was the best option, but it can only be prescribed in pill form so prozac was the back-up plan if DS could not take the pills. He REALLY struggled with the first couple of pills, but by the 3rd dose he knew exactly how to get it down quickly and he is absolutely fine with it and prefers it to liquid medicines (which he almost always thinks taste bad).