OK. I'm going to go w/ Anthony on this -- if you are looking to have kids in the near future I'd probably say spend some time building muscle w/ a good diet that supports muscle building and a solid training program. Have the kids and then worry about the drugs later. It is your call but just to 'get some muscle" - usually very lean people aren't really eating to build muscle, so they are just lean. Lifting for a long time is also one thing, but if it isn't in conjunction with a diet geared towards building mass, it won't necessarily result in mass.
That said - if you choose to pursue some steroid use - stick w/ anavar at doses starting at 5 mg / day - run that for 1.5 - 2 weeks just to make sure there are no weird side effects (e.g the stuff you have could be faked, bad, she may be sensitiive to it - whatever - just being conservative w/ it). Then up it to 10 mg / day and she can run that for 10-16 weeks. Be sure you understand the side effects to expect - e.g. loss of her period (but that doesn't mean she stops ovulating), acne, mass gain, could be some water retention, etc. Some people are sensitive to the increase in blood pressure and get bloody noses. If you don't like the sides then just end the cycle. When the stuff clears (var has a 1/2 life of about 3 weeks if I recall), her natural menstrual cycle and everythign else should return to normal. Also note that steroid use can cause some women to get vaginosis / yeast infections if they are prone to them. To help deal w/ this include acidophilus as part of the supporting dailiy supplements. And just watch for the typical signs of a yeast infection and see the doc (OB/GYN) for an antibiotic to deal with it. And probably a good time to come off the cycle.
Hope that helps. Var is a very mild drug and a lean person should see results with it fairly quickly and be able to maintain those results. But with the above list of caveats. Please be sure to have HER read up as much as she can on anavar so she understands what is involved.