It helps if the bags have a zip closing and a few pockets. A recycled school backpack works great for this purpose. Veto any items that have sharp corners though. This will save an enormous amount of time spent arguing and begging for this trinket or that. Clearly, explain ahead of time, that when spending money runs out, there will be no refill.
Kids are always better shoppers when they are aware of how much of their money they are spending.
Besides their individual bags, a family bag is necessary. This should include wet wipes, not just for baby but for everyone, a few snacks, water, Kleenex, change for vending machines and toll booths, maps, or an atlas. Identification for everyone in the vehicle should also be kept, with contact telephone numbers at home, destination, and any pertinent medical information that may be needed. A few diversions should also be included. A new toy, even the dollar store variety, will offer a few peaceful minutes.