What is good arena dirt?

I live in La Pine Oregon and have well enough "arena" size lot thingy behind our house I will be cleaning out when the ground thaws and am able to get anything harmful out. We have pumice here and I absolutely hate it but what would be nice soil to use for outdoor arena? How deep? I will be training my 4 year old in it, doing trail, possibly reigning, lessons, practicing patterns for competitions. What do you think? I mainly go on trails and try to find open areas with no trees (which is so hard) or I got to others houses that are far far away and gas is getting expensive so I want to make this work.