We do not recommend you do so. Betterment only supports accounts for individuals the ages of 18 or older. If a minor should be appointed as a beneficiary of an account, Betterment may require court-appointed custodial documentation or an official guardian for the minor beneficiary prior to any funds being transferred. Setting a minor as a beneficiary may cause unforeseen delays.
Note that Betterment can support trust accounts where a minor has been designated as the beneficiary. However, the trustee of the trust must be at least 18 years of age.