Substitute for Thai Red Curry Paste?

Could try yellow curry paste or massaman curry paste. They tend to be milder than red curry paste. You can also reduce the amount of the red curry paste used. The problem is that it's not just the chiles that gives off the 'heat', there's also the ginger, the galangal, and the lemongrass. Each contributes to the spicy heat of a curry paste. You could make your own red curry paste and adjust the amount of chiles to your own (or wife's) heat preference. The hard part would be getting some of the ingredients, especially if you live in an area with limited access to an Asian market.