If DS is aged 4 I wouldn't start with Warhammer - I'm saying this as a bloke who has collected and painted the stuff for 20 years, so I know what it's about. Warhammer figures are quite expensive, so unless you are getting into very detailed modelling and painting, and you probably need to be at least 8 years old to do it justice, the unpainted and unassembled models are expensive.
Also, the models are either metal (durable but at 28mm high easy to put in your mouth!) or plastic and needing specialist glue to put them together, and the job would be beyond a small child. And they could break them quite easily! (I have nightmares about my DS aged 3 getting hold of a model I have spent 6 hours painting and breaking it into bits!).
As a former 4 yr old boy I would suggest plastic soldiers of about 1/32 scale (i.e. about 2 inches high or 50mm, give or take a bit) which don't need assembling. 1/72 scale (c. 25 - 28mm) will be a bit small. I think airfix might need assembly, so they will be easier to have bits broken off, and the plastic for self-assembly kits is usually harder and more brittle.
I had a look around on the web, and putting "plastic soldiers" into ebay yields good results, e.g. this, which gives you 80 for £4 in two different colours - bargain!
Cheers,
Permanentvacation
(a self confessed wargaming anorak)