In order to get rigorous stretch in the inner thigh, lie down on the floor with your hips pushed against the wall and legs falling apart and away from each other.
Ask someone with light weight (a small kid) to place his/her feet in a slanting way close to your inner thighs. Once the placing is secured, contract your thighs. To intensify it further, pull your thighs into the groin to activate your inner thighs; then push your legs close to each other while your partner is stretching it apart (opposing forces are active). The person standing on the inner thigh gets slightly lifted up with that push and comes back to the original position with the release of the push. Repeat few rounds of this practice with complete engagement of your core, back and thigh. Make sure that you do not hold your breath. Cooperate with your partner by exhaling deeply. This will optimize the release.
This practice will not only make the inner thigh flexible but also strengthen it by working against the resistance.
This is an extreme test of resistance so make sure you do not have an existing injury and you start with light weight. You may begin with hand press, then one leg before proceeding with both the legs of your partner.