Yes, Landish marine collagen products are safe for children (if they are not allergic to fish) but it is not recommended to exceed Health Canada's range of 2.6-10 g per day. Also, consensus among pediatric nutritionists is that a child requires approximately 1 g of protein for every 2 lbs or 1 kg (2.2 lbs) of body weight daily—collagen, which is protein, should be included in this calculation.
References:
Health Canada (2018). Drugs and Health Products, Hydrolyzed Collagen.