A system, comprising: one or more computing devices of a virtualized computing service; wherein the one or more computing devices include instructions that upon execution on or across one or more processors cause the one or more computing devices to: launch a compute instance at a virtualization host, wherein a portion of memory of the virtualization host is allocated to the compute instance; segregate a first subset of the portion of memory for a first child isolated run-time environment of the compute instance, wherein network communications from the first child isolated run-time environment with any endpoints outside the virtualization host are prohibited other than through a communication intermediary established at the compute instance, and wherein the first subset is inaccessible from programs running outside the first child isolated run-time environment; cause an attestation of a configuration of the first child isolated run-time environment to be initiated by a security manager established at a virtualization management component of the virtualization host; provide, to one or more destinations, (a) a result of the attestation and (b) an indication of an identity of the security manager; determine that the result of the attestation has been accepted; obtain, at the first child isolated run-time environment, an encrypted security artifact via the communication intermediary established at the compute instance, wherein decryption of the security artifact requires a key that is inaccessible to (a) the communication intermediary and (b) other programs running at the compute instance; and perform, at the first child isolated run-time environment, one or more computations using the security artifact and the first subset of the portion of memory. one or more computing devices of a virtualized computing service; wherein the one or more computing devices include instructions that upon execution on or across one or more processors cause the one or more computing devices to: launch a compute instance at a virtualization host, wherein a portion of memory of the virtualization host is allocated to the compute instance; segregate a first subset of the portion of memory for a first child isolated run-time environment of the compute instance, wherein network communications from the first child isolated run-time environment with any endpoints outside the virtualization host are prohibited other than through a communication intermediary established at the compute instance, and wherein the first subset is inaccessible from programs running outside the first child isolated run-time environment; cause an attestation of a configuration of the first child isolated run-time environment to be initiated by a security manager established at a virtualization management component of the virtualization host; provide, to one or more destinations, (a) a result of the attestation and (b) an indication of an identity of the security manager; determine that the result of the attestation has been accepted; obtain, at the first child isolated run-time environment, an encrypted security artifact via the communication intermediary established at the compute instance, wherein decryption of the security artifact requires a key that is inaccessible to (a) the communication intermediary and (b) other programs running at the compute instance; and perform, at the first child isolated run-time environment, one or more computations using the security artifact and the first subset of the portion of memory. launch a compute instance at a virtualization host, wherein a portion of memory of the virtualization host is allocated to the compute instance; segregate a first subset of the portion of memory for a first child isolated run-time environment of the compute instance, wherein network communications from the first child isolated run-time environment with any endpoints outside the virtualization host are prohibited other than through a communication intermediary established at the compute instance, and wherein the first subset is inaccessible from programs running outside the first child isolated run-time environment; cause an attestation of a configuration of the first child isolated run-time environment to be initiated by a security manager established at a virtualization management component of the virtualization host; provide, to one or more destinations, (a) a result of the attestation and (b) an indication of an identity of the security manager; determine that the result of the attestation has been accepted; obtain, at the first child isolated run-time environment, an encrypted security artifact via the communication intermediary established at the compute instance, wherein decryption of the security artifact requires a key that is inaccessible to (a) the communication intermediary and (b) other programs running at the compute instance; and perform, at the first child isolated run-time environment, one or more computations using the security artifact and the first subset of the portion of memory.