Difficult question to give a definitive answer partly because the extent of a 'Duty of Care' can only properly be determined by a Court. That said, the main driver in this area of the law is the Occupier's Liability Act (OLA) 1957 as amended, which is codified civil law. The occupier is not necessarily the owner of the land, but could be the person who occupies or controls the land. If you are deemed the 'occupier' of the land which backs on to the unprotected river then I believe you would have a DoC for visitors and alike.

I suggest you look at what controls could be put in place, which might be signage, barrier, lighting, etc. Notwithstanding signage might be considered a reasonable intervention for an adult, but not a young child. There is a significant amount of case law which others might refer to, but be warned that case law per se is not definitive as each case will be judged on its own individual circumstances.

In criminal law any duty which arises is most likely to be pursuant to the Health and Safety at Work etc Act 1974, section (3). - 1. Where the employer has a duty to ensure, so far as is reasonably practicable, that persons not in his employment who may be affected thereby are not thereby exposed to risks to their health or safety.

If the river bank is not your responsibility and you do not have any control over it, I doubt you could be held liable for anybody falling in.

Whilst "occupier" is not defined in law, the generally accepted definition is "one who has sufficient control over the premises to the extent that he ought to realise that lack of care on his part can cause damage to his lawful visitors". You appear not to have any control over the river bank, and are not in a position to take control measures to prevent falling in.

As there is a tow path between your boundary and the river, as I read it, people cannot "exit your gate directly into the river". They are perfectly safe at the point they exit your property, but if they subsequently fall in because the occupier of the tow path has failed in their duties, that shouldn't be your issue.

Setting aside the fact that it is water, the occupier of a building alongside a busy road wouldn't be expected to put controls in place once people have left their premises onto the pavement.

Why is there access to the river? It’s not yours and not you responsibility unless you are granting free aces when it might be come you responsibility under the law of negligence. Basically it depends on the sort of clients that you have. If they are physically fit and full responsible then access to the river bank should not be a problem. But if they have physical or mental issues which creates a foreseeable risk of them falling in then obviously you should bar access. It is down to you to make that decision.