The gift of shares to your children could have inheritance tax and capital gains tax implications. The valuation of the company is specialist work and (probably) unless the company is listed will be challenged by the Inland Revenue District Valuer anyway. You can get a post transaction valuation check done by HMRC to avoid a later tax return inquiry.
The gifts of shares to your son and daughter would be treated as disposals at market value - which as has been mentioned will need to be agreed with HMRC. Your accountant may be able to do a rough and ready valuation and apply for a post transaction valuation check.
I am sure this would help...