We ( my 5-year old son and I ) took another father/son duo to see "Monster Trucks - IN REAL LIFE" - the kids were not disappointed and are still talking about it now. ( Truth be told the dads were entertained as well ).
If it were logistically more available, I'd go to a larger venue as the trucks never really got to show their full power, but as a first show it was great for the kids.
The promoters really know their audience and the show is packed with lots of movement, lights, music, etc ( and LOUD - bring headphones or earplugs ).
The merchandise was expensive, but not relative to other big ticket shows - e.g. t-shirts $25 which is a lot, but much lower than most concert tees. Our biggest expense was a sno-cone served in a Monster Jam mug for $10 - again, expensive, but not outrageously so.
Tickets were not expensive - we were pretty high up - $20 adult / $5 kids -- I did not search for any deals although I have since seen some referenced.
Overall I recommend the Monster Jam events for good value and very family sensitive/focused entertainment.