Zam's answer was spot-on.
As others said, honesty is key. You don't want to burn bridges, so doing it the way Zam explained makes the most sense.
If you come across as being angry, or -- even worse -- that you are trying to extort the team ("if you don't take kid #2, kid #1 will fly"), that's not good. But every organization, no matter how political will understand simple common sense if it isn't practical for your family.
You: "Did you have any players decline placement? If so, could you please reconsider my child for a spot? It is too difficult for us to get kids to two different rinks, so I will most likely have to decline my other child's offer. We don't want to do that, though, because my child really wants to play here. If you aren't able to offer my child a spot, I totally understand and can appreciate that."
Something like that.