The basic methods for increasing the bed size are:

• Buying a king-size bed.

• (my favorite) Moving the bed out and putting the mattress on the floor.
   This way, more matresses can be attached to the central one and make
   a bed as large as needed.

• Placing extra mattresses on the floor, for the older kids.

• Attaching the baby's crib to the parents bed and removing the crib's
   side next to the bed (this is only a short term solution)