Zenmo uses a couple of methods to link to your bank account. As they are owned by PayPal I assume they are about as secure as PayPal.
Another Venmo dad here chiming in. I have since started Venmoing birthday presents and such to my adult son, which is very handy once your child gets to a certain age. I can just do the last minute, “Here, take the kids out for ice cream or have some fun.”
Update: Venmo is the solution I used and it is fast and easy. Thanks.