These logs are meant to be used with Duplo pieces.  They snap on well (assuming your printer settings are similar to mine).  I designed these some time back to be used with Lego Duplo trains as possible cargo.  They can definitely be used for other things you want to build, so feel free to choose the size you wish to use.  It took me a little while to figure out ideal tolerances for these, so thank you for checking out my models.

They should print fine in a vertical position (base on the plate), using PLA, and without any supports, but if you are having adhesion issues (especially relevant on the taller logs), you should consider using a brim.  

Disclaimer:  Lego Duplo is not meant for small children as they can pose a choking hazard.  There are models amongst these logs I have created that are especially small, so please use your best judgment to decide if your child is ready for something like this.