I would seriously think about building something that will work for life right now, rather than building now and having to build again down the road. While 10x6 will work for awhile it's probably not much different work-wise to do something like 16x8 or 16x10 that's more of a long-term answer.
Are you planning on doing a permanent water feature, something he can fully submerge in when adult? I'm not sure I would, they don't need it and it would be a ton of work to keep sanitary.
ummmmm hmmm i really would make it that big but i dont have the room =( and besides im probly gunna be moving in a year or two so ill have more room just done have it right now
The mulch will be OK and you might want to experiment with something else down the road. Stay away from the pine and the red crap though, plain ol' cypress is the choice.
Yeah if you're planning to move then sure, build it a bit smaller. 10x6 will be fine for quite awhile anyway.