There are likely required distributions, so you need to get this figured out. I'm going to page the best expert on the topic, @dmertz
I agree with the others that someone needs to be appointed guardian or custodian for the child -- presumably that could be you if you are the child's parent. The court could also appoint an independent financial advisor but they will take a fee off the top to administer the account.
I'm going to assume this happened in 2025. If the grandmother was past the age where she is required to take RMDs, and did not take her RMD for 2025, the beneficiary must withdraw and amount equal or more than the grandmother's RMD and include it in their taxable income for 2025. Then, the beneficiary must withdraw their own RMD starting in 2026. If the grandmother was not past the age to take RMDs, the beneficiary does not have to make a withdrawal this year (but can if they want to), but does need to start taking withdrawals in 2026. The beneficiary generally has 10 years to withdraw all the money and close the account. (The rules might be different here, which is why I paged an expert.)
Also, the tax issues for withdrawals are complicated for a minor. Until the child turns 19 (or possibly 24, if they go to college) their withdrawals may be taxed at a higher rate due to the "kiddie tax". If the account is of significant size, you may want to hire a financial and tax planner to figure out the best strategy to pay the least tax, depending on the child's age, other income, status as a student, and marital status.
