How do a Refit the Chain onto a 7 Speed Malvern Star Bike

The only reason I can think of for needing that info is a broken or replacement chain. In either case one needs the proper tool and connecting link to size and install the chain. Do not do so without knowing the proper procedure. Find a bike co-op or bike shop locally to help, or a knowledgeable friend, or Google "install bike chain".

I 'm not an exprt on bikes but the chain depends on the gears. If gears are same, you can use the same chains. So, all you should check if chain fits gears.

I suggest you google for fitting a new bike chain and you will find loads of instructions and videos though having another bike to copy might be the best way to success.

It is not the chain routing you will find difficult I speculate, but finding the right length of chain to fit. The chain you have bought will probably need a few links removing and then once fitted the ends will need rivetting together securely.
You will find that a simple matter when you have seen it done. Adjusting a chain length by adding or removing links is difficult without a chain link extractor tool. Such a tool is inexpensive and an essential addition to the tool box of anyone concerned with chain driven vehicles or equipment.

Thicker chains have the benefit of being able to fit split links and half links but even if they are available they should not be used with a derailleur gear.
Good luck!

your front derailleur should have two small screws for high and low.adjust them so that the derailleur isnt moving to far over.only adjust the screw a half turn at a time.

Google "bicycle shifting technique", for example:
http://coachlevi.com/cycling/complete-beginner-guide-to-bicycle-gears-shifting/
You should NEVER run cross-chained (big-big or small-small). The drive train performs best when the chainline is as close to straight as possible.
Excess sideways stress on the chain will wear it out prematurely and decrease its normal efficiency.
To demonstrate this for yourself, put the bike in the same combination of gears you're having a problem with. Get behind the bike and look straight along the top of the chain. As you can see the line of the chain is a mild zig-zag where it is parallel to the bike at each gear but needs to angle across the drivetrain in between. On some bikes the chain might even come in contact with the next larger chainring when used this way.
The big-big combo uses up all available chain length and the rear derailleur will be pulled toward the front of the bike, sometimes so far that it is in danger of being snapped off.
THERE IS NO UPSIDE TO CROSS-CHAINING.
As you can see, the rear derailleur is probably not long enough to **** up all the slack this gear combination creates. There are other combinations that would yield the equivalent gear ratio without either stressing the drivetrain or dropping the chain.
The basics of shifting are these:
1) The big ring and right side of the cassette in back are for speed.
2) The small ring and the left side of the cassette are for high torque, lower speed
Ancient bike wisdom for beginners: If your legs ache, shift down. If your lungs ache, shift up.

You should go to a bike shop, they can sell you a master link, and keep your own chain. Is your grandson's bike a 1 speed or a multi speed bike, if it is multi then you need a chain tool. Bike shops can do the job cheaper than you can buy the tool and it will be done right. Take the chain back to the store that didn't carry the tool.

The chain has come off. Refit it onto the cogs then you will have drive back.

Vote for me if this helps, thanks.

http://www.parktool.com/repair/readhowto.asp?id=64

You should be able to go to a bike shop and they would be able to cut a length to size for your bike if not I have had luck lawn mower shops they do carry chains