Just here to say that, “Because I said so”, is a perfectly legitimate parenting phrase. 
Somewhere along the way, some pop psychologist told all us parents that we had to explain our every move.  I’m here to tell you…that’s not true.  You DO NOT have to justify your reasoning to someone who doesn’t even reach your kneecaps.  What’s more, they don’t really care.  Even if they shouldn’t get the Happy Happy Hippo with glowing feet and a tummy that rumbles because they just got a Spotted Sally Strawberry Stroller last week and we wouldn’t want to spoil them…blah blah blah, and they can’t have everything they want…blah blah blah, and mummy can’t afford it… blah blah blah.
Yeah, they don’t understand all that responsible jibber jabber or care.  They just want it with every fiber of their adorable little bodies which once they don’t get it will switch with the speed of lightening from adorable to monstrosity in sixty seconds flat.  Their little adorable faces will screw up into wrinkled ugliness that is red and furious.  Those adorable little feet will start kicking you smartly, fiercely in the shins.  So next time before the littlest Hulk’s transformation even starts to begin walk slowly away from the toy aisle repeating surely but steadily, ” Because I said so…because I said so… because I said so….”  They might not like it but they sure do understand it and if they don’t get it the first time they will certainly pick up on it by time number four or five.  Practice makes perfect.
. P.s. This works and is fully acceptable for teenagers as well.  You don’t have to justify why you don’t want to drive them to Timbuktu to meet their friends for five minutes then chauffer them to their next venue all the while filling requests for money and song orders.  Quite frankly, no one should have to justify why they don’t want to do that.  No one wants to do it.  For those of you who don’t have teenagers yet you might think you might want to do this but you don’t.  Why?  Because I said so.  Period.