Hey tjcezar, sorry for the late reply! I'm not sure you have the length for a halfpipe 3' tall with 3' platforms on both sides, check out the ramp calculator at
http://www.xtremeskater.com/ramp-plans/ramp-tool/, if you use the defaults ( 6' 10" transition
radius, 8' flat bottom ) then input ramp-height = 41.5" ( 3' transition-height + 5.5" flatbottom ) and platform-length = 36", the ramp is 26.36 feet. I've no experience but I'd guess that with young kids you want a less steep transition, which makes the ramp even longer. I would recommend reducing the height a little and / or making one of the platforms shorter.