The money was taken from a bash for about 200 kids, including youngsters treated at Yorkhill Hospital For Sick Children, Glasgow.
The 14th annual party, organised by charity fundraiser Les Hoey, was held at Fir Park, Motherwell, on Sunday, as reported in yesterday's Evening Times.
Children were given gifts and entertained by celebrity guests, including SPL footballers and BBC presenter Judith Ralston.
But Mr Hoey and his volunteers were left "distraught" after a bag containing £300 in pound coins, which were collected as part of a raffle, was swiped.
Two mobile phones and a set of keys belonging to a volunteer helper were also stolen.
Mr Hoey, from Wishaw, who works throughout the year to organise trips and treats for sick children, said: "We were all distraught when we realised the money was gone.
"The volunteers spent about two hours searching and hoping to find the bag. They also dialled the numbers of the phones to try and locate them, but they were nowhere to be found."
He added: "All the children who were invited to the party left with arms full of gifts.
"People had given so generously in the raffle because they knew every penny would have been spent on helping these children.
"They way I see it, that £300 would have gone towards buying Beyonce tickets for kids who are going through treatment.
"Nothing like this has ever happened in the 14 years I have been organising these parties."
A police spokeswoman said: "Inquiries are continuing."
Anyone with information on the theft is asked to call police on 101 or Crimestoppers on 0800 555111.