I would be very, very careful with this. You need to ask him why he wants you to do this? As far as I know, if you give him legal guardianship, he will have the power to make ALL the decisions regarding your two children and if he decides they should live with him, he will be able to take them. You would then have to abide by his decisions, whether you like them or not. I am not a solicitor, but before you do anything like this, I would suggest you go and see your local CAB, which you can find here :
http://www.citizensadvice.org.uk/index/getadvi ce.htm
and ask for advice. If necessary they will put you in touch with a solicitor who deals with these matters. Whatever decision you make, it must be in the best interest of the children; but I am sure you already know that. This may also just be an attempt of intimidation on the part of your ex. Either way, I wish you the best of luck with this. :o)