If its a growing savings bond then yes they will know. If they add money to it each month they will know if its been taken out and cashed in. If its a one time deal that was set up that just drew interest then they may know if they contact the bank. You can ask your financial institute if they will know if you want to keep it a secret.
if its just one savings bond that mutures with interest NO they won't know but just a warning they tax the hell outta them. We cashed 500 dollars of mutured savings bonds last year to help with our downpayment on our car and then when we did our taxes we had to pay taxes on it which was like 40% of it so it made like no sense waiting for them to muture and grow if we lose it when we pay taxes. Therefore I have learned savings bonds are a ripoff if someone wants to do something like that they are better off opening a high yield savings account for you then giving savings bonds as gifts like my grandparents did.