With child trust accounts, investing 拢250 initially what is the likeihood of increased money over 18 years?

with an investment.

If the money is invested wisely, then there is a 99% chance of increased money.

You could put the entire 250 into inflation-indexed government bonds. Their return isn't very high, but they are essentially risk-less. Your 250 will appreciate at like 1 or 2% a year for the entire 18 years.

A smarter move is to invest some of that 250 into very safe bonds, another portion into mutual funds (more risk, more return) that generally increase in value, and another portion into stocks (most risk, most return).

If you have a trust account, then there must be a trustee to invest the sum. So long as the trustee is making wise investment choices, the money is almost guaranteed to increase.

I say almost - nothing in the financial sector is ever, ever, ever, certain.

At best, it will double every 7 years or so. I you add money that money will also double every 7 years

this does not seem to be a very good questions
ok
you can buy a book and clarify the meaning of different investment and related stuff like laws, at least one thing is easy and certain, high risk mean high return
but good brokers can make high return high risk into high return mid or low risk
that worth study and interesting