If they are too soft, they may be cause your hands to slip around on the bars causing blisters. Try some ODI Longneck Softies. Most kids love the ODI Longneck grip and the Softy version has a softer feel to it.
You might technically be able to get oversized bars to fit IHC by using the IHC shim and the HIC shim, but that would create a very unstable system. I do not recommend going with this method. If you want to use oversized bars, you need a standard fork and HIC or full-size SCS. If you want to use IHC, you need standard diameter bars.
By "Proto double clamp," I assume you mean the Half-knuckle clamp. You want a slit in your bars for IHC, ICS, and HIC. You only want NO SLIT if you are using SCS. Since the Half-knuckle is not an SCS, you would want the slit in your bars.