Answer
To help children in Uruguay retain Russian, speak Russian with them regularly and consciously support the language at home. If this is not done, children will gradually stop speaking Russian.
The Spanish-speaking environment quickly becomes dominant: school, friends, everyday conversations, and local activities will draw a child toward Spanish. Russian therefore will not be retained automatically just because the parents speak Russian.
Family communication rules, reading, correspondence with relatives, Russian-language activities, and a cultural environment that parents consistently foster all help. If the family wants the child to know Russian culture and speak Russian, it needs to make this a regular practice rather than an occasional reminder.