At the ages of 5 and 8 your children are capable of expressing their own views regarding the issue. If the matter was before the Court, the judge would likely give some weight to their views, but would consider that the children would not have a full appreciation of the risks that accompany whitewater rafting.
Whitewater rafting is perceived as an adventurous activity but online sources indicate that it has a good safety record.
The age limits vary between providers, with some allowing 5-year-olds to participate, and others restricting participation to 16 years and over.
Whether or not the rafting is suitable for young children appears to depend on the river and season (with higher water levels in Spring making rafting more dangerous). Issues that arise with young children include the fact that they can become cold quickly (increasing the risk of hypothermia) and that they may not yet be confident swimmers.
My views are that:
This is an important guardianship matter affecting the children and your ex-husband should be consulting with you prior to making any arrangements;
Your 5-year-old is likely to be an inexperienced swimmer, and possibly still reliant on arm bands or similar. It is not unreasonable for you to be withholding your consent to going white water rafting.
White water rafting may be suitable for your 8-year-old if a suitable river and white water rafting class is chosen and they are accompanied by a fully certified and experienced guide. However, arguably these activities can wait until your children are a bit older. The issue is subjective and opinions are likely to vary.
I suggest you notify your ex-husband in writing (email is fine) to confirm that you do not consent to the children participating (or perhaps, consent only to the 8-year-old participating if your husband can satisfy you that an appropriate level of rafting has been chosen with a safe provider).
You should provide a timeframe for him to respond, say within 48 hours. Given the whitewater rafting has been planned for next week, if you do not receive an adequate response from your ex-husband you will need to file an urgent application in the Family Court.
If you and your ex cannot agree on the issue then the decision will need to be made by a judge in the Family Court. As noted above, the children’s wishes and views will be accorded limited weight in this situation given their ages. The court will, of course, give serious consideration to the safety of the activity.