ISA: doesn't matter who earned the money, just stick within the annual limit of £20k. If you ever move the money to another provider for a better interest rate, make sure you transfer it via the special process, don't just withdraw it and try to pay it in.
Pension: consider a stakeholder pension. They have low fees and a low minimum monthly contribution. You can pay in even if you don't work although I think there's an annual limit regarding the tax/Govt contribution.
YY to what a PP said about keeping up with NI contributions.
Children's accounts: the inheritance tax thing only applies if the 'giver' has given away over a certain amount each year, and that tapers off as the years go by, and it only applies to the 7 years before they die. So unless they're giving your kids thousands of pounds a year, it probably won't be a problem. Any tax would be due from their estate, not from the children's saving accounts. The money would no longer be owned by the GP so they have no right to get it back if they want/need it in future.
The only problem with children's accounts is that once they are old enough to withdraw the money, they can do so and spend it on whatever they like. So you won't have any control over them spending it on something worthy and not frittering it away.