Do you really need grippers and a carpet kicker?

[Lusi83]

I have been given some carpet from my new neighbour....it is from the equilivalent room so already cut to the right size for the room and been down for a while (spare room so not worn). It is hessian backed and seems good quality.
The carpet we are replacing was here when we moved in, it is hessian backed but poor quality. We lifted it to decorate the room...
there is (old) underlay down but no grippers...it had been tacked down around the edges. I hired a knee kicker and put new tacks around the edges...10 yrs ago.
Over the years it developed a crease in the middle- it had been pulled up a couple of times to access underneath and not restretched and the underlay is past its best...as is the carpet.

So do I really need gripper rods? Do I really need to stretch it? (It is £12 a day to hire a kicker...or I can get a new one from B&Q for £23 -which I guess would help if it needs pulling up again)
Should I buy new underlay...?

Didn't really want to spend too much on this -it is for a child's room so likely get ruined..(current one has red nail varnish and sparkly glue on it) but then I don't really want the new one to crease up

 

[mointainwalker]

Why are you worried about it possibly creasing if it is:

a) In a child's room ( I'm sure he/she won't be bothered)
b) You accept there is a good chance it will be ruined anyway .

The above being especially pertinent as "..

Didn't really want to spend too much on this

 

[Lusi83]

Don't want it to crease because it is a pain to vacuum...also that is where it is worn the most

Think I probably didn't ask the question very well...
What will be the most likely cause of the creases - not restretched after lifting, carpet tacks not holding it down well enough or just because the underlay (and carpet) are knackered?
Could carpet tacks alone work?
I need to lift some (good) carpet on the landing (really creaky floor boards) - so I'm thinking if I really need to stretch it when putting it back down the £23 kicker might not be a bad investment...