The 36-year-old "Sweet Home Alabama" actress and husband Jim Toth welcomed their little boy Tennessee James into the world last month.
The royal couple, who Witherspoon met when they visited Los Angeles last year, were among the first to send a gift and a handwritten note, reported Femalefirst online.
Witherspoon'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 USD 2,000.
"Rob was one of the first to call and he even promised unlimited babysitting services. The baby is the toast of Hollywood," a source said.
Witherspoon has two children, Ava, 13, and Deacon, eight, from her marriage to Ryan Phillippe.
