There are a number of things you need to be taking into account such as how long you intend to have the account open for (children's accounts tend to go up to their 18th or 24th birthday), or if you'll need to access this at some point before then, etc.

Having spent a long time looking into this for my own kids I've become familiar with this minefield! - Look into what is on offer first, and outweigh the rates with penalty charges for early withdrawals and the like.

I came across this whilst doing my research and found it very useful, hope it helps:

http://www.moneyfacts.co.uk/savings/guides/guides-child-accounts.aspx

I have some cash to invest for my kids and I'm wondering if there is a savings account with a reasonable rate of interest for around £15K (for minimium 5 years) , which requires the signature of both myself and my ex-wife to withdraw ? or should I be considering some kind of trust ?

Basically I'm happy for the money to be jointly invested but we would be both happier if there was a safeguard to stop either partnr clearing out the account.

Cheers