Either plan a head and start saving about $100 a month when they are 10 or borrow the money, such as a loan against insurance, second mortgage etc.
Make a budget and stick to it. Don’t have a fancy wedding, have it during the week when it’s cheaper. Ask about the package deals hall, photographer, florist & band for one price. Ask the other parents to chip in.
As far as supporting the newly weds if you are having problem spaying for the wedding which is $7,000 – $20,000.00 on the low side, I don’t think you are in a position to support them financially and you should explain it to them.