I've been through this and my daughter is currently going through it. Firstly, please get legal advice and don't rely on our well-intentioned advice here: I've spotted a few things - for instance, it may be different if you are in Scotland and not England.
I am not encouraging any breaking of the law. There is a difference between that and making your own position as favourable as you can. If your son does any jobs he bills for, he might want to think about the timing of submitting his invoices. When the couple submit their financial documents these will be for a specific period, so it can be advantageous to think about what goes in and out of the accounts in that time - it can be seem very unfair what is considered 'income'. If someone owes him money and wants to repay, for instance, hold off so it doesn't appear on his statements and then be counted as 'income'. Provision for the less able spouse and for the children are separate considerations. I'm guessing you'd rather your son was in a position of not giving his wife loads of money but would want the children to be well-provided for (I could be wrong of course). Since none of us know the individuals concerned, we shouldn't say that anything that goes to the wife will benefit the children. She may drink it for all we know. I'd get your son to keep any receipts for his expenses with the children, if he buys them clothes/shoes/pays for classes etc. so they can be factored in.
Don't give your son any money to save, invest, or buy him an expensive capital good like a car - it will be counted in the assets. I'd give cash when you see him so he can use if for shopping, petrol etc as others have said. Cash isn't traceable (again not wishing to break any laws).
I was the more vulnerable spouse but my daughter is the stronger one ( having married a waste of space), so I do try to see 360 degrees. I do get a bit cross when money from a house sale is considered a 'pot of money coming to you' as it may well, as in my case, be intended to furnish my later life as I don't have much of a pension etc. The solicitor will tell you how far in advance these inheritances can be considered relevant. If you and husband are only 68 for example, why should a possible inheritance in 25 years' time be considered? it will probably have to go on your care home anyway!