The 'Sweet Home Alabama' actress and husband Jim Toth welcomed their little boy Tennessee James into the world last month and the royal couple - who Reese met when they visited Los Angeles last year - were among the first to send a gift and a handwritten note.
Reese's 'Water for Elephants' co-star Robert Pattinson, who hid out at her California ranch following his break-up with Kristen Stewart, also sent a lavish gift, a cradle from Posh Tots worth almost $2,000.
A source told America's Star magazine: ''Rob was one of the first to call and he even promised unlimited babysitting services. The baby is the toast of Hollywood.''
Reese - who has two children, Ava, 13, and Deacon, eight, from her marriage to Ryan Phillippe - previously joked she was upset not to be invited to William and Catherine's wedding as she had asked them to go to hers.
Speaking before the pair married, she said: ''I can't even contain my enthusiasm for the royal wedding! I'm kind of bummed I'm not invited to the royal wedding. My girlfriend asked me if I was invited. Well, no, but I invited them to my wedding!''