I have three kiddos, my oldest is 4 going on 5 and has asked many times to have her own playhouse on the place..and since the Little
House on the Prairie are her favorite on the shelf she will only settle for a log cabin. Took her out yesterday for her first few hours of cutting wood and spent most of it looking at wild flowers and deer. A few questions for those of you with extensive cabin experience- I am doing butt and pass corners, do you prefer the long 12-14" screws or traditional spikes to connect the logs? It will only be 8'x8' with a little porch off the front so not too big of spans to deal with. Also should I put it up on a treated post foundation or right on the ground as I really only expect it to be played in for the next 5-7 years.

When she out-grows the cabin, if built correctly, you may find other uses for it, so I would build it right. Depending on log size, you could stack the first row on treated 4"x material. I would simply use galvanized spikes throughout, pining the first course to the 4x material, and end-spiking the log end together. Pre-drill for the spike, of course using a slightly smaller bit than the spike. Structural screws are very expensive, also.

AK vikings advice about building on PT base is spot on, additionaly, debark the logs if you want longevity.
I know everyone want to see logs on the interior, but if you observe Scandinavian and Russian log construction (the experts and originators of log construction), you will see that most of their interior walls are furred out, often with insulation between the logs and the interior surface. This is something to consider.

X3 about doing it right the first time.
I would also consider making it a bit bigger for your own future use as a whatever..
For pinning the logs: I much prefer to use wood for the pins. I like to drill a one inch round hole down through two courses of logs and drive a one inch square peg into that hole. The peg is larger when measured across corners . If you use a hardwood peg those corners cut into the softer wood of the log.
Use a drift pin to get the top of the pin below the surface of the log.

This allows you to cut openings wherever you want without running your saw into a spike or a screw. It also allows the logs to settle without the steel screws or spikes preventing it or pushing up between logs

Probably not necessary for a play house or storage or whatever after she outgrows it. But I've stayed in log cabins/houses that are just logs, and ones that are furred out and insulated, the difference in both summer and winter is unbelievable. Logs are just not very good insulation. They have a reputation for being good insulation, but I believe that is because back in the old days they were comparing them to board houses with nothing, or at most a simple air space between walls as insulation.

Also, rounded logs on the inside tend to collect dust, spider webs, etc. and are a pain to clean. And not nearly as easy to attach things, from hanging pictures, to cabinets. If you aren't going to cover the logs inside, I would recommend squaring them off on the inside, to facilitate cleaning and interior decorating/modifying. Again, possibly not necessary for a play house, but something to think about if you plan to keep and use it.