The child seat laws state that children up to 135cm or 12 years old must be in a suitable child restraint where there is one fitted and usable. What you are suggesting sounds fine. If you cannot fit a booster in the middle then it is not a problem, I cant get three kids boosters/seats across the back of my td5.
If you dont have 3 point seatbelts it does not mean you cannot put kids in the car, likewise if it is an oldie and has no belts it is still ok for kids to travel in it.
Basically if safety belts are fitted you must make use of them with suitable child seats/boosters where possible to fit them.
The following is from latest edition of highway code:
Children under 3 - correct child restraint MUST be used. If one is not available in a taxi child may travel unrestrained
From 3rd birthday up to 1.35M tall or 12yrs old- correct child restraint MUST be used where seat belts are fitted. MUST use adult belt where child restraint is not available in a taxi or private hire vehice, or if fitting of two occupied restraints prevents fitment of a third
child over 1.35m or 12yrs - seatbelt MUST be worn if available
Hope that helps.