Im going to disagree with the majority by saying it depends on what expectations have been laid out.
At the end of the day its the parents money. No one has to pay for a wedding and MN folks are always banging on about dont be entitled etc, no one owes you anything, so it depends on context.
I.e. If youve never specifically offered to pay fpr the wedding, nor said anything about it and now your child is getting married you can say 'It would mean a lot to me if this specific thing happened therefore I will pay for it.' Its then the recipients choice but take the choice with good grace and dont hold it over them.
If you have offered the money and are now placing demands thats not ok. If you have always said you would happily contribute to their dream wedding and are now adding goalposts then thats not ok.
Weddings are emotional and a lot, so all sides have my sympathy in this. The best way to do it is set a time, assume everyone coming to the table is doing so with the best intentions and lay out the issues. 'we are delighted to contribute to your special day but by insisting that you get married at the top of mount everest we wont be able to attend and we love you so much and want to see you married, perhaps there could be a blessing and party at home?'
Or
'youve always said you would give me the same amount you gave my siblings. Im hurt that because ove chosen to get married in a barn you are saying you will pull the money unless we move to the country club. This is why i want the barn and can you explain your reservation about the barn.'
Open communication because everyone wants the day to be perfect.