Almost certainly not, because the rules of language change still apply. Without considerable engineering (whose methods have probably not been invented yet, or at least not been tested on a large scale and refined), it is likely that there will be a lot of corners that are subject to relatively quick change when people start to use the language.
Similar situations occur in practice which demonstrate this in the variant "no matter how easy the original language actually is". In certain contact situations, a very simple pidgin can develop and become so successful that many children grow up with it as their native language. Within a few generations it becomes a full creole language.
There is no reason to assume that one wouldn't see similar instability issues in the situation you describe, where the new language is complex rather than simple. (Or with a new language of average simplicity.) It's important to understand that the evolution of languages doesn't converge towards a regular, logical and practical state but is caused by factors such as people's desire for succinctness, vivid expression, politeness etc.
Note that complications in the original language are definitely not the issue in the way you seem to be worried about them. If the language is easy enough for the first generation to learn perfectly, then it's definitely easy enough for their children to pick up.