I bought the LS400 when our daughter was 3 months old - Great car but it was a bad move. We had a Range Rover before and going from something with this level of flexibility to a 3 box saloon was, in hindsight, madness.
The Lexus is not flexible (nor was it ever designed to be) for the usual paraphernalia of pushchairs, travel cots, sterilisers, high chairs etc. etc. that you need when taking on holiday.
The LS is also horrific in the snow and ice - to the point that I shelled out on a cheappie 4x4 just for the winter months. The fact is that I still have it as it just suits the whole family lifestyle and can cope with all the luggage and tat that you throw at it.
The LS was never designed as a family car so I am not knocking the product in any way - Don't do it, it really doesn't work. I have some comical pictures of the LS before we left for a weekend (yes just a weekend!) in center parcs. I even had things on my lap, the journey was misery for all concerned.