Can I use my assets to pay CSA?

I left my wife in April 2012 and now live with my new partner and her little girl.

I paid privately until June this year maintenance for my 2 boys who live with my x One was 18 in July thus year the other us 15 I paid £250 per month from mat 2012 until June 2013 along with phone bills a car and car insurance for my eldest son, but due to financial difficulties along with ill health had to stop payments.

The house my x lives in has an equity amount in it of £135000.00 half of which is mine which I am wanting to make an agreement in writing that when the youngest us 16 I will get £10.000 the remainder is left as maintenance money for the youngest and then balance is split between the 2 boys when the time comes.

Please advise.