Agents will be able to see in the record that your account supported the upgrade, but the passengers will only see "upgraded" and the number of PlusPoints used when they look at the website or app reservation.

As the others noted, there is potential for confusion when doing things anonymously, so you may consider letting your family know somehow to avoid them being surprised and having an awkward conversation when checking in, for instance. Also, wouldn't it be better to have them know someone in the family is being generous, rather than mistaking it for some benevolence from United?