Tom weighs 5kg more than Harry. Harry weighs 3kg more than Freddie who. in turn weighs 2kg less than Alfie. What is the difference, in kilograms between Tom and Alfie?

6kg?

Tom weighs 5kg more than Harry.
H+5=T
Harry weighs 3kg more than Freddie
H-3=F or H=F+3
who in turn weighs 2kg less than Alfie.
F+2=A

What is the difference, in kilograms between Tom and Alfie?

T-A=H+5-(F+2)
Then substitute H for F and finish it.

I think it's too hard for a 10 year old fwiw.

I suppose could do it by drawing a line:

Freddie ----- 2----- Alfie ----- 1 ------ Harry ------- 5 -------- Tom

Is there any easy solution for 10 years old?

Hermanmunster’s suggestion is a good one.
Drawing diagrams really helps with probkems lije this. In school children use white boards for rough working and diagram/pictorial representation is encouraged

If you give Alfie a nominal weight of, say, 10kg then work backwards, it is quite straightforward:

Alfie = 10kg

Freddie = Alfie -2 = 8kg

Harry = Freddie + 3 = 11kg

Tom = Harry + 5 = 16kg

So Tom - Alfie = 16 - 10 = 6kg

(At these weights the boys would be babies!!)

DS2 was 19.3Kg at his weighing in Reception - I only remember that because I noted at the time that it was exactly 5 times his birth weight .
.

_________________
Outside of a dog, a book is a man's best friend. Inside of a dog it's too dark to read.Groucho Marx

I would do it as a vertical list.

So, Freddie weighs 2kg less than Alfie:

Alfie
-2
Freddie

Harry weighs 3kg more than Freddie, so Harry goes 1kg above Alfie:

Harry
-1
Alfie
-2
Freddie

Tom weighs 5kg more than Harry, so that can easily be fitted at the top:

Tom
-5
Harry
-1
Alfie
-2
Freddie

So, the difference between Tom and Alfie is 5+1=6, so 6kg.

This is effectively an introduction to inequalities. It is best explained using number lines. as suggested earlier.
if you draw the child a line and location Tom, Harry and Freddie...it becomes fairly simple.

The real difficulty is to explain to the child that similar questions may require the equation treatment when it involves equality and the relationship are relative (multiple) rather than absolute (add/subtract).