Cover them well at night with several layers of fleece, old net curtains, newspaper or whatever you have. The tunnel saves one degree of frost, the propagators maybe another degree, but tender plants like morning glory may be sensitive to cold as well as to actual frost. Is it possible to take them into the house?
My windowsills indoors are overflowing with flower seedlings, plus some tomatoes, because it's far too soon to put them up in the tunnel here. I did put some shasta daisies and lettuce in the greenhouse which is inside the tunnel, but I can re-sow those if they get frosted. Where we are I assume there will be frosts until early June, but I tend to take some plants up to the greenhouse in May then protect them if there is a hard frost due. It's quite a juggling act to get seedlings ready early enough that they will flower or crop before the autumn frosts, but not so early that they succumb to the spring frosts
and to give them sufficient light that they aren't drawn and pale.
I have a heated propagator with a plant light as well with all the delicate things in like most of my tomatoes, butternut squash and chillies. Haven't sown the cucumbers yet.