In our gym/program we start all kids from day 1 in competitive to aim to get to as square hip splits as possible. Sadly there is a degree of acceptance. We demand and expect more from the more natural flexible kids, and make amends and adjustment with those that aren't (everyone however gets down).
With our young ones we put them between boxes so that they can hold themselves up vertical a lot easier, and also as coaches we constantly correct all kids, explain to them, show them and everything else inbetween. So far i have never come across a kid who has not understood what is required of them, however willingness to achieve does greatly differ between kids.
At 6 i think its important to start them aiming for squareness as this is a prime age to develop flexibility at, and it does make squareness a much easier task at that age.
One thing that we have found is that practicing square splits tends to have a negative effect on leaps, because they try and hit same position in leaps, which becomes really hard.we accommodate this by working all dynamic work with a turned out back leg, which compensates for this.
All and all i am in total favor (personally) to work on squareness from the start, but do have realistic expectations given the kids natural ability.