Obviously don't malnourish them but I wouldn't recommend the rack At 25% they won't stick out. Being very tall or very short can be a nuisance - people thinking you're older or younger than you are as a child for example.
Best thing you can do is encourage them in lots of self confidence. Taller people can get noticed more which can help in life. (I think statistically speaking managers for example are taller than average.) So if you get your DC to be noticed because they're nice, interesting and confident they won't care they're not tall.
And maybe get them to take up gymnastics where being short is an advantage rather than say basketball!
My DS was small as a child and shorter than his peers. He's now the tallest in our family having shot up at puberty. He's still stick thin though so buying trousers is a nightmare. So I wouldn't worry just yet what will be will be.
Now this is purely anecdotal - before anyone has a go at me - but in my family there is no one above 5ft 6 apart from my brother who is 6ft 2. I believe that the answer is full fat milk - my mother would make him drink at least a pint of milk every day until he was about 16/17.
Luckily my dd loves milk and I encourage her to drink her as much as possible ( about a pint and half of full fat milk a day) and she towers over me - although her height can be explained by her dad who is 5ft 11!
I know not all children like milk and there are lots of other issues that people may have with milk ( we try to stick to organic - £3 for 8 pints from most supermarkets ) but that is my twopenny worth!
I was tall as a child, at 11 I was the tallest in my primary school. I am now 5 ft 1. Only 1 inch taller than I was at 11. I don't think there is much you can do to influence overall height (unless there is a medical need for growth hormones for those who are simply not growing at all) nor do I think there is a need to do so anyway.
DS is 5 and slightly smaller/lighter than your 5 year old but I don't see it as an issue at all. He is who he is and as long as he is eating well and developing well then thats all that matters
My DS has always been short, usually skirting the blue zone at the bottom of the height chart (0.4th centile). I realised quite early on that I couldn't influence his height, so worked on things I could affect. He's now 13, 140cm, and very confident and self-assured.