Firstly, to forestall the police doubting permission, make sure that they get WRITTEN permission from the land owner. Does the land owner know that they are there? Has he actually given permission for the kids, or just to your friend. If he has only given permission to your friend, and hasn't specifically said he can give permission to others then your friend has no right to give others permission, and so they are there illegally.
As airsoft guns are very low powered (I know it doesn't feel like it when you take a hit on the finger at close range but they are) they generally fall outside the provisions of the various firearms acts (these apply to "lethal, barreled weapons" and anything as low powered as an airsoft gun is not considered "lethal") most of the restrictions on age etc. do not apply. If the guns are considered realistic, then they might have to show how they got hold of them, but possession itself is not illegal.
The limitations about pellets crossing boundaries only applies to air weapons that are classed as firearms, but it's never a good idea to fire weapons so that the projectiles end up where you can't see, and where you don't have permission to be, as it asks for complaints to be made. Whether there is any offence committed, it's NOT fun looking down the barrel of a very real, very loaded MP5.
As for contacting your local firearms enquiry officer, I'd steer clear. You are not dealing with a firearm, and they will inevitably try to impose conditions on you which have no basis in law, or public safety. If you must, inform them that you will be carrying out a lawful activity on the land, but don't give them the chance to stick their oar in. Most are ok, but some will try to spoil it. I'm sure that FEO isn't a job from someone who is rabidly anti-shooting, but it would appear that some most certainly are.
So if everyone has proper permission, and you are sensible, then no offence will be committed. If you start shooting at passers by, running along the fencelines with your RIF's on display and generally being twats then expect to be arrested and to receive a good b########g at the very least. They may well seize the weapons, for testing to ensure they are legal. They won't be able to legally seize them permanently, as long as no offence has been committed, but it's amazing the damage a gun can suffer rolling around the boot of a police car. The coppers don't care for your guns, expect to get them back damaged. I've seen £2000 full bore rifles come back from seizure with scratched stocks and barrels, and even an air rifle with a bent barrel. Now I'm not saying the police would deliberately damage property as some kind of unofficial sanction, and knocks and scratches could well happen if a gun is placed un-bagged in the boot of a car, but a bent barrel???