in my opinion, childrens isas are rubbish - there's usually not much benefit in having tax free savings as most children don't make enough interest to pay tax on it anyway. If circumstances change at all over the next 18 years, you can't access the money under any circumstances as it belongs to the child the moment it goes in the account. There is also no discretion over when they get it - it's their 18th birthday (so for example if they needed it when they were 17 tough luck, or if they are feckless at 18 there's nothing to stop them receiving the money and going out and spending it all on cocaine, or a tattoo or a Fosters up holiday)
What about getting some premium bonds for them? or just a regular savings account that you can add to?
i have just opened a children's savings account for my daughter with lloyds and it's currently paying 3%, and i retain control of it, so if we needed the money i could get to it