A minor can own property including copyright.
A minor can also enter into contacts that are genuinely in their interest - note that this is an additional protection not afforded to adults. A minor needs the permission of their legal guardian to enter a contract and while this can be implicit in your circumstances it would be wise to make it explicit.
Notwithstanding, a minor may revoke a contract (if possible - they can't if they have drunk the milkshake, for example) while they remain a minor and for a reasonable time after they attain their majority. If they do this you would lose the copyright and they would have to refund your money. This is just a risk you will have to bear and factor into what you are willing to pay.
As an aside, there are three "properties" your contract needs to deal with: any physical artifacts (like a painting), the copyright and the moral copyright.