it is their choice to have children, even though they are young - why not have kids now - my brother and his wife had a kid when they were 20/21 - they are young now and can enjoy a life that those in their 30's with young kids can't - and they had very little money too..why not enjoy every minute of it.
OP, money is always going to be an issue - you have managed up to now to save for a house on relatively low salaries...
With a bit of bugeting and planning you can save a few pounds - we are in the process of doing this ourselves - pay off all loans asap as you are paying interest... As for staying at home you might find it tough on one income, even a part time job or child minding can bring in some extra money..
All the best,
P