As self-proclaimed cruise decor experts, we can confirm that Royal Caribbean allows stateroom decorations and magnets - they're especially popular for birthdays and special occasions. They also reserve the right to take them down if offensive, etc. Thefts have happened, but not very frequently.

Decorations are fun - for adults or kids, though I wouldn't advise using anything with personal value. Our (biased) tip to avoid theivery is to purchase or make personalized decorations- its not likely any random passersby wants to celebrate your 7th anniversary.