8 weeks of use ad-libitum. Parents will be encouraged to have their children play Mightier games at least 3 times a week (totalling 45 minutes or more play each week) for the 8-week duration. Parents in the Child Play condition will be advised specifically to not play Mightier for the 8-week duration of the study.

8 weeks of use ad-libitum. Parents will be encouraged to have their children play Mightier games at least 3 times a week (totalling 45 minutes or more of play each week) for the 8-week duration. Additionally, parents in the Child and Parent Play condition will be encouraged to play Mightier games at least once a week (totalling 15 minutes or more of play each week) for the 8-week duration.

Mightier is a video-game-based biofeedback intervention that capitalizes on children's love of video games to increase emotional awareness and facilitate emotion regulation practice through heart rate (HR) control. Each family receives a Mightier Kit (Mighty Band heart rate monitor, dedicated Mightier tablet unless the family prefers to use their own device) and the Mightier App. Children wear a "Mighty Band" heart rate monitor on their arm while they play any one of 26+ games. As their heart rate rises the games become more difficult. For example, during a cooking game, smoke may appear on the screen and obscure the player's view. Children can either opt into an explicit cool down activity (deep breathing, progressive muscle relaxation, crossing the midline, or visualization) or cool down on their own. By simply playing Mightier, children are motivated to practice calming strategies in moments of challenge.

Behavior and Feelings Survey (BFS): The BFS (Weisz et al., 2014) is a 12-item scale measuring child behavior and emotion problems. The child's parent is prompted to read the survey questions to the child and to note the child's responses. The researcher will screen share the BFS form and record child responses in the form. Children will complete the BSF at baseline and Week 8. Items are rated on a scale from 0 (not a problem) to 4 (a very big problem). Three scale scores can be derived: Internalizing Problems (sum of items 1-6), Externalizing Problems (sum of items 7-12), and Total Problems (sum of items 1-12).

Affective Reactivity Index-Parent Report (ARI-P): The ARI is a 7-item scale that consists of 6 symptom items and 1 impairment item. The scale was designed to determine irritable mood rather than behavioral consequences such as hostility and acts of aggression (Stringaris et al., 2012). The individual items are scored 0,1, 2, and only the first six items are summed to form the total score, with a minimum score of 0 and a maximum score of 18, and higher scores indicating greater severity of irritability symptoms. The seventh item is an impairment item and it is analysed separately, with a minimum score of 0 and a maximum score of 3, and higher scores indicating greater irritability symptom severity. Parent participants will complete the ARI-P at baseline and Week 8.

Global Improvement Scale (GIS): The GIS is a parent self-report question that asks parents if they have noticed any overall improvements in their child's emotion regulation, with response options ranging from (1) very much improved to (7) very much worse. Minimum score = 1, Maximum score = 7, with higher numbers indicating less improvement, or worsening. The GIS has not been scientifically validated. Parent participants will complete the GIS at Week 8.

Difficulties in Emotion Regulation Scale, 18-question short form (DERS-18): The DERS-18 (Victor & Klonsky, 2016) is an 18-question measure of the respondent's emotion regulation experiences. For scoring, three responses are reverse-coded then all responses are summed, resulting in an overall score and scores on 6 subscales: Awareness, Clarity, Goals, Impulse, Nonacceptance, and Strategies. Higher scores indicate greater emotion regulation difficulties. Parent participants will complete the DERS-18 at baseline and Week 8.