Here's Trey Parker's technique for getting a good performance while recording the voice of toddler Ike Broflovski for South Park. In this case, Canadian adoptee Ike is voiced by none other than Trey's daughter Betty Boogie Parker.
The technique is similar to the method Adam McKay used to coach Pearl in The Landlord:
According to Ike's Wikia fan page entry, Ike has been voiced by about a dozen children and adults over the years, including Matt Stone and Bill Hader.
In case the Instagram post above doesn't work, here it is ported to YouTube: