As others suggest, I have never been able to get an intra-Europe seat assignment ahead of time. I have always had the seat I prefer assigned at OLCI or physical check in. Never an exception. Although in Voyager domestic I'm ST Platinum and am always on an international Business reservation. Friends who often travel with infants report complete success as long as they've a ST Elite in the family, which they do.

It's frustrating to be unable to select in advance, but we always get what we want anyway so we're not so concerned anymore. OLCI ASAP, though, so you can select while the selecting is good.