How are you related to the estate, are you a conservator, a guardian, a personal representative, a trustee? How is the minor child involved? There may be an issue of court approval depending on the authority which you the right to sell. I would consult a local probate attorney, bring your documents, lay out all the facts for him, and see what he says.
The information provided is based solely on the general information given and should not be construed as legal advice for your specific situation.
There is no set percentage. Generally you are only required to sell for the fair market value. If it is less than FMV, I would make sure that you are selling to a bona fide purchaser (e.g. no related party or interested person). If you are, there could be issues depending on numerous other facts that you have not disclosed and we would have no way of knowing. I would consult with an attorney to confirm that you are acting in the beneficiaries best interest before getting too far down the road with any sale.