hello we have a special educational needs group that uses a room with a push barrier fire exit and a fire rated entrance  door, some children have been getting out of the fire rated entrance door as it is not locked this has happened 3 times. Are we ok to lock the fire door if staff members all have a key as our primary means of escape to outside is still our fire exit

Keys are usually not acceptable except in exceptional circumstances such as lawful places of detention.

A similar premises I have dealt with secures the fire exits where the students who are at risk of absconding with magnetic locks. These are linked to the fire alarm to release and in place of the usual green break glass overrides are key operated switches, the fire alarm break glasses are provided with hinged covers to prevent misuse and the fact that the alarm releases the locks is not advertised, the keys are used instead. The premises are also sprinklered.

You should ask your fire safety adviser or risk assessor to look into this and produce a risk assessment that justifies a suitable solution. Consultation with the fire and rescue service may be advisable.

Your suggestion may end up being accepted, but as it would be exceptional you need to go through the right process to justify it.

Have you considered fitting a DOOR ENTRY ALARM SET with off/on switch, high up the door which will not prevent the children using the door but will alert you if they do and when they realise they cannot sneak out quietly they may not try. 

so having the room fire exit still available would not mitigate locking the entrance door

It would not, both doors are fire exits, doors required to escape from a fire. If the fire was in an adjacent classroom and smoke/flames prevented you using the final exit door to outside then you would need to use the other door, it all depends on the layout of the premises.

I would suggest you do what AB suggested you should ask your fire safety adviser or risk assessor to look into this and produce a risk assessment that justifies a suitable solution. Your suggestion may end up being accepted, but as it would be exceptional you need to go through the right process to justify it. Consultation with the fire and rescue service may be advisable.