I would have a few questions that don't really need to be answered but just for thinking purposes.
1) Who gives the child the money in the first place?
2) Is the money in a bank account or is this cash?
3) Does the child "earn" the money?
4) What is the goal or purpose behind giving the child money?
My ex-husband and I are very friendly, but money is something that we never saw eye to eye on. Therefore my "allowance" for the kids is completely separate from the "allowance" or other reward money that they have at their dads house.
For my kids (boys 11 and 9) I have set up a rather elaborate money saving plan. The reason I have done this is mostly because my oldest son has a large sum of money that he will be getting when he turns 18, due to a lawsuit against a daycare that I have shared previously on these boards. My goals are to teach the boys to save alot of money and forget its there, and then manage their budget to live on the remainder.
Here is what I do. They get 5$ per week or $20 per month each. They never see the real cash. $6 or 30% is automatically put into savings accounts that I have set up for them individually. The remaining $14 is put on to Pre-paid credit cards. (I have all of this set up automatically online with automated funds transfers.)
I hold their credit cards at my house in their wallets with any cash gifts from family and friends they get during the holidays, birthdays etc. They recieve monthly statements for their savings accounts and we track their savings and credit cards in Quicken (accounting software for personal use basically a fancy check register).
The boys are not allowed to spend the money in their savings accounts. They must budget and manage their credit card and cash to be sure that they have enough for birthday and christmas gifts and their own personal wants.
I would suggest that you protect your son's money. If you are giving him money to save and spend then set up some rules. What are your goals for his money? What are you trying to teach him?
You and your ex-wife are setting some life long precidences with his money habits and spending. Loaning people money (or taking out a loan yourself) is a serious risk financially and should be treated with great caution at any stage in life. He needs to learn that in my opinion.
Not to be rude, but your ex-wife is an adult and should manage her money better so that she doesn't need to take money from her son.