When in doubt go with what the I-751 instructions say (uscis.gov/I-751):
If you have dependent children who acquired conditional resident status on the same day as you or within 90 days thereafter, then include the names and Alien Registration Numbers (A-Numbers) of these children in Part 5. of Form I-751 in order to request that the conditions on their status be removed as well. If you have dependent children who did not acquire conditional resident status on the same day as you or within 90 days thereafter, or if the conditional resident parent is deceased, then those dependent children must each file Form I-751 separately to have the conditions on their status removed.