Hallo Swara Kijana,
Most people these days prefer variable power scopes but I prefer (for hunting anything larger than rodents), a simple 4x, with standard crosshairs or crosshairs with small dot.
With the excellent 7x64 Brenneke, if I was sure I'd only use it in open places like the Eastern Cape, the Kalahari, the Namib Desert and similar places, I might consider a 6x scope.
But, if this rifle is for general hunting in other places, I feel a 6x would frustrate me on things like bushbuck, nyala, warthog and many others found in thick bush, riverine forest and such.
I see that you live in the Eastern Cape and I am truly jealous, because I understand it to be a Rifleman's Paradise.
Anyway, for these fixed power scopes, Leupold is pretty good and affordable but if you can find a Zeiss or Swarovski or Schmidt & bender, Khales, etc., they are worth the extra money in clarity and ruggedness.
The Austro-German ones mentioned are unfortunately difficult to find lately, I don't know if they are even being made at the moment (variable power scopes are very much in style now).
I believe Leupold still makes their very good 4x scope today.
The good news is, a used but excellent condition 4x scope can sometimes be bought for a reasonable price, because they often sit on the gun shop shelf, gathering dust for a couple years and the owner might accept a low offer on the price to finally get rid of it.
I have had this good fortune more than once or twice.
Here in the US where I live, most hunters tend to want too much velocity in their rifle and too much magnification in their scopes and people evidently trade in their Grandfather's old 4x scope toward a huge new variable type.
Don't let a 4x scope fool you.
German, Canadian British and American Snipers in WW-II generally used no more than 4x scopes, in many cases only 2 & 3/4 power ones, to consistently shoot people through the helmet, at several hundred meters/yards.
If I could not hit a springbok through the vitals at 400 meters with a 4x scope on a relatively flat shooting rifle such as your 7x64, I could only blame myself for not practicing enough with it.
If I used a large scope in thick forest and a bushbok jumped out of my field of view and back into thick bush at 10 yards, until disappearing, I could only blame myself for buying too much scope.
As for the caliber to get for your 8 year old:
The only experience I have with silencers was on a 9mm MP5 submachine gun so, I cannot comment with any validity regarding putting one on a hunting rifle (I know it is popular in many places, and for good reason no doubt).
Anyway, I would avoid the .270 since it is so very similar to your 7x64 and would complicate your ammunition supply needlessly.
If I wanted to stay within that ballistic category, but with a silencer installed, I might just get another 7x64 and put one on it.
The .243 might be a fun rifle for everyone in the family, including the 8 year old, on certain species.
But, it is a very specialized caliber / not versatile in African terms IMO.
If you do not hand load, I would consider the 6.5x55 caliber for your 8 year old.
Very low recoil but also, it is versatile enough to use on quite a few African species.
I am only a beginner to Africa but have hunted and messed around with rifles constantly since age 16.
The .243 I feel is a bit dodgy on larger animals like kudu and tougher animals like warthog to name but two of the many.
The 6.5x55 might be marginal for these but, a 160 grain bullet vs a 100 grain bullet says it all, (again IMO).
If you hand load, you can select almost any reasonable cartridge and just load it way down for the youngster, until he grows into it.
Sorry for the length of this rant (too much coffee/caffeine this morning),
Velo Dog.