He should have created a spreadsheet showing the split from the start and then deducted their spending and added a % of the investment. Consistent sharing between and if he was going to have an " investors" fee that should have been discussed and agreed upon at the start. He can be reimbursed for the taxes he had to pay on it (but not the purchase amount because it was a gift so that is his cost).

However, the money is his boys'. Just like all toys and clothes legally would belong to the parents, so does the money. Would he say the toys are his or his boys'? He would say "Go pick up YOUR toys." Same with the money...it is not his...never was. The ticket was a gift therefore the winnings are not his.

Bottom line...be a fair dad and dont go back on your word. YTA