Where are you travelling from/to?
How many seats could you manage with?
It is perfectly acceptable to buy a child ticket for an under-5 with a family railcard to avail yourself of the railcard discount, just as it would ordinarily be acceptable to buy a child's ticket for an under-5 so that they are entitled to a seat. You don't need to lie about anyone's age.
If you just buy one adult and one child ticket with your family railcard, it must be cheaper than buying an adult ticket without a discount, because 30% off full price + 60% off half price is less than 1 x full price. The only time this is not true is if 60% off half price is less than £1, as £1 is the minumum price.
A family railcard gives you access to some saver tickets which you may not be able to purchase at that time of day without it, although this is not as bargainous as it used to be, when you could buy a supersaver return for a peak train...
There are often offers on the purchase of a family railcard- google it before purchasing.
On a pendolino there is turning space in the carriage with a disabled seat - we always put our pushchair there, although obviously if someone needs to turn, you would need to move. You also need to be orepared to collapse your pushchair if necessary, although we rarely needed to during our pushchair years.